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.c b/openbsc/src/nat/bsc_nat.c
index f5eca80..ded05c6 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -874,7 +874,7 @@
 			goto exit2;
 			break;
 		}
-        } else if (parsed->ipa_proto == NAT_IPAC_PROTO_MGCP) {
+        } else if (parsed->ipa_proto == IPAC_PROTO_MGCP) {
                 bsc_mgcp_forward(bsc, msg);
                 goto exit2;
 	} else {