correct type of qsort() comparison callback

Originally committed as revision 9338 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/fraps.c

@@ -88,7 +88,8 @@ static int decode_init(AVCodecContext *avctx)
  * Comparator - our nodes should ascend by count
  * but with preserved symbol order
  */
-static int huff_cmp(const Node *a, const Node *b){
+static int huff_cmp(const void *va, const void *vb){
+    const Node *a = va, *b = vb;
     return (a->count - b->count)*256 + a->sym - b->sym;
 }