decoder/ffpmeg: use forward declarations

src/decoder/plugins/FfmpegMetaData.cxx

@@ -25,6 +25,10 @@
 #include "tag/TagTable.hxx"
 #include "tag/TagHandler.hxx"
 
+extern "C" {
+#include <libavutil/dict.h>
+}
+
 static constexpr struct tag_table ffmpeg_tags[] = {
 	{ "year", TAG_DATE },
 	{ "author-sort", TAG_ARTIST_SORT },

src/decoder/plugins/FfmpegMetaData.hxx

@@ -20,15 +20,7 @@
 #ifndef MPD_FFMPEG_METADATA_HXX
 #define MPD_FFMPEG_METADATA_HXX
 
-extern "C" {
-#include <libavutil/dict.h>
-}
-
-/* suppress the ffmpeg compatibility macro */
-#ifdef SampleFormat
-#undef SampleFormat
-#endif
-
+struct AVDictionary;
 struct tag_handler;
 
 void