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/include/osmocom/msc/debug.h b/include/osmocom/msc/debug.h
index 717cf74..28ba482 100644
--- a/include/osmocom/msc/debug.h
+++ b/include/osmocom/msc/debug.h
@@ -21,5 +21,6 @@
 	DVLR,
 	DIUCS,
 	DBSSAP,
+	DSGS,
 	Debug_LastEntry,
 };