SGSN: force GSUP CN domain to PS

Always set CN domain in outgoing GSUP packets to PS to make it
compatible with osmo-auc.

Change-Id: Ia6ee2e55a41a8ea9e465d7df1b2b3559b553fca8
Related: OS#1582
diff --git a/openbsc/src/gprs/gprs_subscriber.c b/openbsc/src/gprs/gprs_subscriber.c
index 71f5ff1..658ce04 100644
--- a/openbsc/src/gprs/gprs_subscriber.c
+++ b/openbsc/src/gprs/gprs_subscriber.c
@@ -165,7 +165,7 @@
 
 	if (strlen(gsup_msg->imsi) == 0 && subscr)
 		strncpy(gsup_msg->imsi, subscr->imsi, sizeof(gsup_msg->imsi) - 1);
-
+	gsup_msg->cn_domain = OSMO_GSUP_CN_DOMAIN_PS;
 	osmo_gsup_encode(msg, gsup_msg);
 
 	LOGGSUBSCRP(LOGL_INFO, subscr,