Fix a "function declaration isn't a prototype" warning.

Originally committed as revision 24480 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mxfenc.c

@@ -77,7 +77,7 @@ typedef struct {
     UID container_ul;
     UID element_ul;
     UID codec_ul;
-    void (*write_desc)();
+    void (*write_desc)(AVFormatContext *, AVStream *);
 } MXFContainerEssenceEntry;
 
 static const struct {