movenc: Change the track struct name to match the typedef

This makes the struct name (which isn't used anywhere) match the
name of the typedef, as for all the other structs declared in this
header.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.h

@@ -73,7 +73,7 @@ typedef struct MOVFragmentInfo {
     int64_t tfrf_offset;
 } MOVFragmentInfo;
 
-typedef struct MOVIndex {
+typedef struct MOVTrack {
     int         mode;
     int         entry;
     unsigned    timescale;