time/ISO8601: forward-declare StringBuffer

src/song/ModifiedSinceSongFilter.cxx

@@ -20,6 +20,7 @@
 #include "ModifiedSinceSongFilter.hxx"
 #include "LightSong.hxx"
 #include "time/ISO8601.hxx"
+#include "util/StringBuffer.hxx"
 
 std::string
 ModifiedSinceSongFilter::ToExpression() const noexcept

src/time/ISO8601.cxx

 /*
- * Copyright 2007-2017 Content Management AG
+ * Copyright 2007-2019 Content Management AG
  * All rights reserved.
  *
  * author: Max Kellermann <mk@cm4all.com>
@@ -33,6 +33,7 @@
 #include "ISO8601.hxx"
 #include "Convert.hxx"
 #include "Parser.hxx"
+#include "util/StringBuffer.hxx"
 
 StringBuffer<64>
 FormatISO8601(const struct tm &tm) noexcept

src/time/ISO8601.hxx

 /*
- * Copyright 2007-2017 Content Management AG
+ * Copyright 2007-2019 Content Management AG
  * All rights reserved.
  *
  * author: Max Kellermann <mk@cm4all.com>
@@ -33,13 +33,14 @@
 #ifndef TIME_ISO8601_HXX
 #define TIME_ISO8601_HXX
 
-#include "util/StringBuffer.hxx"
 #include "util/Compiler.h"
 
-#include <string>
 #include <chrono>
 
+#include <stddef.h>
+
 struct tm;
+template<size_t CAPACITY> class StringBuffer;
 
 gcc_pure
 StringBuffer<64>