avformat/mxfdec: Correct confusing struct tag

Don't use typedef struct MXFTrack {...} MXFTimecodeComponent, in
particular given the fact that MXFTrack is a type of its own.
Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/mxfdec.c

@@ -131,7 +131,7 @@ typedef struct MXFSequence {
     uint8_t origin;
 } MXFSequence;
 
-typedef struct MXFTrack {
+typedef struct MXFTimecodeComponent {
     UID uid;
     enum MXFMetadataSetType type;
     int drop_frame;