avformat/internal: Move muxing-only functions to new mux.h header

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

libavdevice/opengl_enc.c

@@ -54,9 +54,9 @@
 #include "libavutil/log.h"
 #include "libavutil/opt.h"
 #include "libavutil/avassert.h"
-#include "libavutil/avstring.h"
 #include "libavformat/avformat.h"
 #include "libavformat/internal.h"
+#include "libavformat/mux.h"
 #include "libavdevice/avdevice.h"
 #include "opengl_enc_shaders.h"