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_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index 89d7d4b..4fda517 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -260,7 +260,7 @@
 	msg->l3h = msgb_put(msg, length);
 	memcpy(msg->l3h, data, length);
 
-        return bsc_write(bsc, msg, NAT_IPAC_PROTO_MGCP);
+        return bsc_write(bsc, msg, IPAC_PROTO_MGCP);
 }
 
 int bsc_write(struct bsc_connection *bsc, struct msgb *msg, int proto)