sgsn: add a minimalistic ACL

This adds a minimalistic ACL by which certain, individual roaming IMSIs
can be authorized to use the SGSN.  So you can selectively bypass the
'MCC+MNC == first 5 digits of IMSI' checking for a couple of IMSIs
diff --git a/openbsc/src/gprs/sgsn_main.c b/openbsc/src/gprs/sgsn_main.c
index 08e0a85..9d42648 100644
--- a/openbsc/src/gprs/sgsn_main.c
+++ b/openbsc/src/gprs/sgsn_main.c
@@ -75,6 +75,7 @@
 	.config_file = "osmo_sgsn.cfg",
 	.cfg = {
 		.gtp_statedir = "./",
+		.acl_enabled = 1,
 	},
 };
 struct sgsn_instance *sgsn = &sgsn_inst;