filter/route, ...: add missing stdlib.h includes

NEWS

 ver 0.18.4 (not yet released)
 * decoder
   - dsdiff: fix byte order bug
+* fix build failures due to missing includes
 
 ver 0.18.3 (2013/11/08)
 * fix stuck MPD after song change (0.18.2 regression)

src/SongSave.cxx

@@ -30,6 +30,7 @@
 #include "util/Domain.hxx"
 
 #include <string.h>
+#include <stdlib.h>
 
 #define SONG_MTIME "mtime"
 #define SONG_END "song_end"

src/encoder/OpusEncoderPlugin.cxx

@@ -34,6 +34,7 @@
 #include <glib.h>
 
 #include <assert.h>
+#include <stdlib.h>
 
 struct opus_encoder {
 	/** the base class */

src/filter/RouteFilterPlugin.cxx

@@ -56,6 +56,7 @@
 #include <assert.h>
 #include <string.h>
 #include <stdint.h>
+#include <stdlib.h>
 
 class RouteFilter final : public Filter {
 	/**

test/run_convert.cxx

@@ -36,6 +36,7 @@
 
 #include <assert.h>
 #include <stddef.h>
+#include <stdlib.h>
 #include <unistd.h>
 
 static void