avformat/mxf: Use AVUUID for uids

This is in preparation for using av_uuid functions directly.
Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/mxf.h

@@ -25,8 +25,9 @@
 #include "libavutil/log.h"
 #include "libavutil/pixfmt.h"
 #include "libavutil/rational.h"
+#include "libavutil/uuid.h"
 
-typedef uint8_t UID[16];
+typedef AVUUID UID;
 
 enum MXFMetadataSetType {
     AnyType,