avformat/utils: Move ff_stream_add_bitstream_filter to mux.c

It is muxing-only; in fact, it should be considered part of
the core muxing code.

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

libavformat/flvenc.c

@@ -25,13 +25,12 @@
 #include "libavutil/avassert.h"
 #include "libavutil/mathematics.h"
 #include "libavcodec/mpeg4audio.h"
-#include "avio_internal.h"
 #include "avio.h"
 #include "avc.h"
 #include "avformat.h"
 #include "flv.h"
 #include "internal.h"
-#include "metadata.h"
+#include "mux.h"
 #include "libavutil/opt.h"
 #include "libavcodec/put_bits.h"