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/mmst.c

@@ -625,4 +625,5 @@ URLProtocol ff_mmst_protocol = {
     .url_read       = mms_read,
     .url_close      = mms_close,
     .priv_data_size = sizeof(MMSTContext),
+    .flags          = URL_PROTOCOL_FLAG_NETWORK,
 };