avcodec/flac: Don't use bytestream API unnecessarily

It makes no sense here, as flac_parse_block_header()
is not even supposed to advance the caller's pointer.

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

libavformat/flacdec.c

@@ -20,6 +20,7 @@
  */
 
 #include "libavutil/channel_layout.h"
+#include "libavcodec/bytestream.h"
 #include "libavcodec/flac.h"
 #include "avformat.h"
 #include "demux.h"