fix compiler warning: osmo_prim_cb() has void* arg

Change-Id: I16741976050b3f36196af1b4f18d795af2b2722a
diff --git a/src/hnbgw_cn.c b/src/hnbgw_cn.c
index 61db35d..09b2726 100644
--- a/src/hnbgw_cn.c
+++ b/src/hnbgw_cn.c
@@ -327,8 +327,9 @@
 }
 
 /* Entry point for primitives coming up from SCCP User SAP */
-static int sccp_sap_up(struct osmo_prim_hdr *oph, struct osmo_sccp_link *slink)
+static int sccp_sap_up(struct osmo_prim_hdr *oph, void *ctx)
 {
+	struct osmo_sccp_link *slink = ctx;
 	struct hnbgw_cnlink *cnlink = osmo_sua_link_get_user_priv(slink);
 	struct osmo_scu_prim *prim = (struct osmo_scu_prim *) oph;
 	int rc;