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/rtmpproto.c

@@ -1000,4 +1000,5 @@ URLProtocol ff_rtmp_protocol = {
     .url_write      = rtmp_write,
     .url_close      = rtmp_close,
     .priv_data_size = sizeof(RTMPContext),
+    .flags          = URL_PROTOCOL_FLAG_NETWORK,
 };