avformat/rtpdec_asf: fix compiler warning about const qualifier being discarded

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

libavformat/rtpdec_asf.c

@@ -188,7 +188,7 @@ static int asfrtp_parse_packet(AVFormatContext *s, PayloadContext *asf,
 
         av_freep(&asf->buf);
 
-        ffio_init_context(pb, buf, len, 0, NULL, NULL, NULL, NULL);
+        ffio_init_context(pb, (uint8_t *)buf, len, 0, NULL, NULL, NULL, NULL);
 
         while (avio_tell(pb) + 4 < len) {
             int start_off = avio_tell(pb);