nat: Inform others if an IMSI is rejected

In case one wants to monitor the access lists one
there is now a trap for the IMSI.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat.c b/openbsc/src/osmo-bsc_nat/bsc_nat.c
index 6b061bc..2f6711a 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat.c
@@ -1060,6 +1060,8 @@
 			filter = bsc_nat_filter_sccp_cr(bsc, msg, parsed,
 						&con_type, &imsi, &cause);
 			if (filter < 0) {
+				if (imsi)
+					bsc_nat_inform_reject(bsc, imsi);
 				bsc_stat_reject(filter, bsc, 0);
 				goto exit3;
 			}
@@ -1091,6 +1093,8 @@
 					filter = bsc_nat_filter_dt(bsc, msg,
 							con, parsed, &cause);
 					if (filter < 0) {
+						if (imsi)
+							bsc_nat_inform_reject(bsc, imsi);
 						bsc_stat_reject(filter, bsc, 1);
 						bsc_send_con_release(bsc, con, &cause);
 						con = NULL;