Add SGs Interface

Add an SGs interface (3GPP TS 29.118) to osmo-msc in order to support
SMS tunneling and Circuit Switched Fallback (CSFB)

Change-Id: I73359925fc1ca72b33a1466e6ac41307f2f0b11d
Related: OS#3615
diff --git a/src/libmsc/silent_call.c b/src/libmsc/silent_call.c
index b4fc154..2a9fa9c 100644
--- a/src/libmsc/silent_call.c
+++ b/src/libmsc/silent_call.c
@@ -128,7 +128,8 @@
 	 * This doesn't apply to the situation after MSCSPLIT with an
 	 * A-interface. */
 	req = subscr_request_conn(vsub, paging_cb_silent, data,
-				  "establish silent call");
+				  "establish silent call",
+				  SGSAP_SERV_IND_CS_CALL);
 	if (!req)
 		return -ENODEV;
 	return 0;