avformat/internal: Avoid casting const away

Fixes many warnings when using -Wcast-qual.
Reviewed-by: Tomas Härdin <git@haerdin.se>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/internal.h

@@ -418,7 +418,7 @@ static av_always_inline FFStream *ffstream(AVStream *st)
 
 static av_always_inline const FFStream *cffstream(const AVStream *st)
 {
-    return (FFStream*)st;
+    return (const FFStream*)st;
 }
 
 #ifdef __GNUC__