fftools/ffmpeg_mux: Make ms_from_ost() inline

Signed-off-by: softworkz <softworkz@hotmail.com>

fftools/ffmpeg_mux.h

@@ -123,7 +123,7 @@ typedef struct Muxer {
 
 int mux_check_init(void *arg);
 
-static MuxStream *ms_from_ost(OutputStream *ost)
+static inline MuxStream *ms_from_ost(OutputStream *ost)
 {
     return (MuxStream*)ost;
 }