avformat/matroska: add missing Buttons track type

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

libavformat/matroska.h

@@ -271,6 +271,7 @@ typedef enum {
   MATROSKA_TRACK_TYPE_COMPLEX  = 0x3,
   MATROSKA_TRACK_TYPE_LOGO     = 0x10,
   MATROSKA_TRACK_TYPE_SUBTITLE = 0x11,
+  MATROSKA_TRACK_TYPE_BUTTONS  = 0x12,
   MATROSKA_TRACK_TYPE_CONTROL  = 0x20,
   MATROSKA_TRACK_TYPE_METADATA = 0x21,
 } MatroskaTrackType;