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/bsc/osmo_bsc_msc.c b/openbsc/src/bsc/osmo_bsc_msc.c
index 6644982..0bdb71f 100644
--- a/openbsc/src/bsc/osmo_bsc_msc.c
+++ b/openbsc/src/bsc/osmo_bsc_msc.c
@@ -68,7 +68,7 @@
         }
 
 	mgcp->l2h = msgb_put(mgcp, ret);
-	msc_queue_write(data->msc_con, mgcp, NAT_IPAC_PROTO_MGCP);
+	msc_queue_write(data->msc_con, mgcp, IPAC_PROTO_MGCP);
 	return 0;
 }
 
@@ -225,7 +225,7 @@
 		}
 	} else if (hh->proto == IPAC_PROTO_SCCP) {
 		sccp_system_incoming(msg);
-	} else if (hh->proto == NAT_IPAC_PROTO_MGCP) {
+	} else if (hh->proto == IPAC_PROTO_MGCP) {
 		mgcp_forward(data, msg);
 	}