osmo_rat_type: add OSMO_RAT_EUTRAN_SGS

osmo-msc is about to implement the SGs interface and requires a RAT indicator
for that.

Change-Id: I00588396bfe03feba38ecb0717d584594f0b2b46
diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c
index 3862047..9eb41fb 100644
--- a/src/gsm/gsm_utils.c
+++ b/src/gsm/gsm_utils.c
@@ -1003,5 +1003,6 @@
 	{ OSMO_RAT_UNKNOWN, "unknown" },
 	{ OSMO_RAT_GERAN_A, "GERAN-A" },
 	{ OSMO_RAT_UTRAN_IU, "UTRAN-Iu" },
+	{ OSMO_RAT_EUTRAN_SGS, "EUTRAN-SGs" },
 	{}
 };