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/msc_ifaces.c b/src/libmsc/msc_ifaces.c
index 3074d07..e2c52df 100644
--- a/src/libmsc/msc_ifaces.c
+++ b/src/libmsc/msc_ifaces.c
@@ -28,6 +28,7 @@
 #include <osmocom/mgcp_client/mgcp_client.h>
 #include <osmocom/msc/vlr.h>
 #include <osmocom/msc/a_iface.h>
+#include <osmocom/msc/sgs_iface.h>
 #include <osmocom/msc/gsm_04_08.h>
 #include <osmocom/msc/msc_mgcp.h>
 
@@ -60,6 +61,10 @@
 		msg->dst = conn->iu.ue_ctx;
 		return ranap_iu_tx(msg, 0);
 
+	case OSMO_RAT_EUTRAN_SGS:
+		msg->dst = conn;
+		return sgs_iface_tx_dtap_ud(msg);
+
 	default:
 		LOGP(DMSC, LOGL_ERROR,
 		     "msc_tx(): conn->via_ran invalid (%d)\n",