remove warning, key is const

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@7509 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/mxf.c

@@ -141,7 +141,7 @@ typedef struct MXFContext {
     UID content_storage_uid;
     MXFMetadataSet **metadata_sets;
     int metadata_sets_count;
-    uint8_t *sync_key;
+    const uint8_t *sync_key;
     AVFormatContext *fc;
 } MXFContext;