avio: fix "discards const qualifier from pointer target type" warning

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

libavformat/avio.c

@@ -302,7 +302,7 @@ int ffurl_write(URLContext *h, const unsigned char *buf, int size)
     if (h->max_packet_size && size > h->max_packet_size)
         return AVERROR(EIO);
 
-    return retry_transfer_wrapper(h, buf, size, size, (void*)h->prot->url_write);
+    return retry_transfer_wrapper(h, (unsigned char *)buf, size, size, (void*)h->prot->url_write);
 }
 
 int64_t ffurl_seek(URLContext *h, int64_t pos, int whence)