decoder/ffpmeg: don't use av_metadata_conv() in ffmpeg 0.7

It's a no-op and deprecated.

src/decoder/ffmpeg_decoder_plugin.c

@@ -568,7 +568,9 @@ ffmpeg_stream_tag(struct input_stream *is)
 		: 0;
 
 #if LIBAVFORMAT_VERSION_INT >= ((52<<16)+(31<<8)+0)
+#if LIBAVFORMAT_VERSION_INT < AV_VERSION_INT(52,101,0)
 	av_metadata_conv(f, NULL, f->iformat->metadata_conv);
+#endif
 
 	for (unsigned i = 0; i < sizeof(ffmpeg_tag_maps)/sizeof(ffmpeg_tag_map); i++) {
 		int idx = ffmpeg_find_audio_stream(f);