ipaccess: Put our extensions to the protocol into the same enum

Rename NAT_IPAC_PROTO_MGCP to IPAC_PROTO_MGCP and place it in
the enum. We need to be prepared to change this number if IPA
is ever going to use it for something else.
diff --git a/openbsc/src/nat/bsc_filter.c b/openbsc/src/nat/bsc_filter.c
index e968fa2..9c09a83 100644
--- a/openbsc/src/nat/bsc_filter.c
+++ b/openbsc/src/nat/bsc_filter.c
@@ -70,7 +70,7 @@
 	{ IPAC_PROTO_SCCP, ALLOW_ANY, ALLOW_ANY, ALLOW_ANY, FILTER_TO_BOTH },
 
 	/* allow MGCP messages to both sides */
-	{ NAT_IPAC_PROTO_MGCP, ALLOW_ANY, ALLOW_ANY, ALLOW_ANY, FILTER_TO_BOTH },
+	{ IPAC_PROTO_MGCP, ALLOW_ANY, ALLOW_ANY, ALLOW_ANY, FILTER_TO_BOTH },
 };
 
 struct bsc_nat_parsed *bsc_nat_parse(struct msgb *msg)