avformat/rtpdec_qt: Fix 'warning: passing argument 2 of ffio_init_context discards const qualifier from pointer target type'

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/rtpdec_qt.c

@@ -71,7 +71,7 @@ static int qt_rtp_parse_packet(AVFormatContext *s, PayloadContext *qt,
      * http://developer.apple.com/quicktime/icefloe/dispatch026.html
      */
     init_get_bits(&gb, buf, len << 3);
-    ffio_init_context(&pb, buf, len, 0, NULL, NULL, NULL, NULL);
+    ffio_init_context(&pb, (uint8_t*)buf, len, 0, NULL, NULL, NULL, NULL);
 
     if (len < 4)
         return AVERROR_INVALIDDATA;