filter/registry: move `extern` lines to plugin headers

src/filter/FilterRegistry.cxx

 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -19,6 +19,9 @@
 
 #include "FilterRegistry.hxx"
 #include "FilterPlugin.hxx"
+#include "plugins/NullFilterPlugin.hxx"
+#include "plugins/RouteFilterPlugin.hxx"
+#include "plugins/NormalizeFilterPlugin.hxx"
 
 #include <string.h>
 

src/filter/FilterRegistry.hxx

 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -30,10 +30,6 @@
 
 struct FilterPlugin;
 
-extern const FilterPlugin null_filter_plugin;
-extern const FilterPlugin route_filter_plugin;
-extern const FilterPlugin normalize_filter_plugin;
-
 gcc_pure
 const FilterPlugin *
 filter_plugin_by_name(const char *name) noexcept;

src/filter/plugins/AutoConvertFilterPlugin.cxx

 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -22,7 +22,6 @@
 #include "filter/FilterPlugin.hxx"
 #include "filter/Filter.hxx"
 #include "filter/Prepared.hxx"
-#include "filter/FilterRegistry.hxx"
 #include "AudioFormat.hxx"
 #include "util/ConstBuffer.hxx"
 

src/filter/plugins/NormalizeFilterPlugin.cxx

 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -21,7 +21,6 @@
 #include "filter/FilterPlugin.hxx"
 #include "filter/Filter.hxx"
 #include "filter/Prepared.hxx"
-#include "filter/FilterRegistry.hxx"
 #include "pcm/PcmBuffer.hxx"
 #include "AudioFormat.hxx"
 #include "AudioCompress/compress.h"

src/filter/plugins/NormalizeFilterPlugin.hxx

 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -22,8 +22,11 @@
 
 #include <memory>
 
+struct FilterPlugin;
 class PreparedFilter;
 
+extern const FilterPlugin normalize_filter_plugin;
+
 std::unique_ptr<PreparedFilter>
 normalize_filter_prepare() noexcept;
 

src/filter/plugins/NullFilterPlugin.cxx

 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -24,10 +24,10 @@
  * plugins.
  */
 
+#include "NullFilterPlugin.hxx"
 #include "filter/FilterPlugin.hxx"
 #include "filter/Filter.hxx"
 #include "filter/Prepared.hxx"
-#include "filter/FilterRegistry.hxx"
 #include "AudioFormat.hxx"
 #include "util/Compiler.h"
 #include "util/ConstBuffer.hxx"

src/filter/plugins/NullFilterPlugin.hxx

+/*
+ * Copyright 2003-2019 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_NULL_FILTER_PLUGIN_HXX
+#define MPD_NULL_FILTER_PLUGIN_HXX
+
+struct FilterPlugin;
+
+extern const FilterPlugin null_filter_plugin;
+
+#endif

src/filter/plugins/RouteFilterPlugin.cxx

 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -39,13 +39,13 @@
  * one of them takes effect.
  */
 
+#include "RouteFilterPlugin.hxx"
 #include "config/Domain.hxx"
 #include "config/Block.hxx"
 #include "AudioFormat.hxx"
 #include "filter/FilterPlugin.hxx"
 #include "filter/Filter.hxx"
 #include "filter/Prepared.hxx"
-#include "filter/FilterRegistry.hxx"
 #include "pcm/PcmBuffer.hxx"
 #include "pcm/Silence.hxx"
 #include "util/StringStrip.hxx"

src/filter/plugins/RouteFilterPlugin.hxx

+/*
+ * Copyright 2003-2019 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_ROUTE_FILTER_PLUGIN_HXX
+#define MPD_ROUTE_FILTER_PLUGIN_HXX
+
+struct FilterPlugin;
+
+extern const FilterPlugin route_filter_plugin;
+
+#endif