nat: Remember if the MSC accepted the connection on principle

For the USSD module we only want to handle requests that are
legitimate. This is achieved by looking for the CM Service Accept
or the BSSAP CIPHER MODE COMMAND.
diff --git a/openbsc/include/openbsc/bsc_nat_sccp.h b/openbsc/include/openbsc/bsc_nat_sccp.h
index 0ade668..4ae640e 100644
--- a/openbsc/include/openbsc/bsc_nat_sccp.h
+++ b/openbsc/include/openbsc/bsc_nat_sccp.h
@@ -77,6 +77,7 @@
 	/* status */
 	int con_type;
 	int con_local;
+	int authorized;
 	int imsi_checked;
 	char *imsi;