sgsn: Do not search the name with the "domain" name attached to it

In case the query for "hostname" will fail c-ares will append the
domain name of /etc/resolv.conf and query again. We don't want that
so claim we provide a list of domain names and then don't provide
any.
diff --git a/openbsc/src/gprs/sgsn_ares.c b/openbsc/src/gprs/sgsn_ares.c
index c4012a4..eabf940 100644
--- a/openbsc/src/gprs/sgsn_ares.c
+++ b/openbsc/src/gprs/sgsn_ares.c
@@ -155,7 +155,7 @@
 	options.sock_state_cb = setup_ares_osmo_fd;
 	options.sock_state_cb_data = sgsn;
 
-	optmask = ARES_OPT_FLAGS | ARES_OPT_SOCK_STATE_CB;
+	optmask = ARES_OPT_FLAGS | ARES_OPT_SOCK_STATE_CB | ARES_OPT_DOMAINS;
 
 	if (sgsn->ares_servers)
 		optmask |= ARES_OPT_SERVERS;