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/gopher.c

@@ -121,4 +121,5 @@ URLProtocol ff_gopher_protocol = {
     .url_write      = gopher_write,
     .url_close      = gopher_close,
     .priv_data_size = sizeof(GopherContext),
+    .flags          = URL_PROTOCOL_FLAG_NETWORK,
 };