change extradata type to uint8_t *

Originally committed as revision 6551 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -741,7 +741,7 @@ typedef struct AVCodecContext {
      * - encoding: set/allocated/freed by lavc.
      * - decoding: set/allocated/freed by user.
      */
-    void *extradata;
+    uint8_t *extradata;
     int extradata_size;
 
     /**