Constify sgsn_acl_lookup() parameter

This requires I414e67a3de733fab407161b3264d3b89070ba537 in libosmocore
to avoid warning about discarded const.

Change-Id: Ie92637dd900b0f9eba891d5aad0b4ba0ee69c08c
diff --git a/include/osmocom/sgsn/gprs_sgsn.h b/include/osmocom/sgsn/gprs_sgsn.h
index cf78766..0b721a3 100644
--- a/include/osmocom/sgsn/gprs_sgsn.h
+++ b/include/osmocom/sgsn/gprs_sgsn.h
@@ -460,7 +460,7 @@
 extern const struct value_string *sgsn_auth_state_names;
 
 void sgsn_auth_init(void);
-struct imsi_acl_entry *sgsn_acl_lookup(const char *imsi, struct sgsn_config *cfg);
+struct imsi_acl_entry *sgsn_acl_lookup(const char *imsi, const struct sgsn_config *cfg);
 int sgsn_acl_add(const char *imsi, struct sgsn_config *cfg);
 int sgsn_acl_del(const char *imsi, struct sgsn_config *cfg);
 /* Request authorization */
diff --git a/src/gprs/sgsn_auth.c b/src/gprs/sgsn_auth.c
index e3d9127..50f2126 100644
--- a/src/gprs/sgsn_auth.c
+++ b/src/gprs/sgsn_auth.c
@@ -43,7 +43,7 @@
 	INIT_LLIST_HEAD(&sgsn->cfg.imsi_acl);
 }
 
-struct imsi_acl_entry *sgsn_acl_lookup(const char *imsi, struct sgsn_config *cfg)
+struct imsi_acl_entry *sgsn_acl_lookup(const char *imsi, const struct sgsn_config *cfg)
 {
 	struct imsi_acl_entry *acl;
 	llist_for_each_entry(acl, &cfg->imsi_acl, list) {