avformat/utils: Move ff_get_extradata to demux_utils.c

It is only used by demuxers (although it is hypothetically
possible that some day e.g. a protocol might need it, but
that is unlikely given that they don't deal with AVCodecParameters).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/mpc.c

@@ -22,6 +22,7 @@
 #include "libavutil/channel_layout.h"
 
 #include "avformat.h"
+#include "demux.h"
 #include "internal.h"
 #include "apetag.h"
 #include "id3v1.h"