avio: Add an URLProtocol flag for indicating that a protocol uses network

This definition is in two files, since the definitions will move
to the private header at the next bump.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/udp.c

@@ -491,4 +491,5 @@ URLProtocol ff_udp_protocol = {
     .url_close           = udp_close,
     .url_get_file_handle = udp_get_file_handle,
     .priv_data_size      = sizeof(UDPContext),
+    .flags               = URL_PROTOCOL_FLAG_NETWORK,
 };