gsup: Add value_string for Session State IE

In Change-Id I1cee271fed0284a134ffed103c0d4bebbcfde2a8 we added support
for a new session state IE, but we didn't add any value_string array
for string conversion of it.  Let's fix this.

Change-Id: I3d9f087786dc37c42498fa9a2be07483ec93ba7b
diff --git a/include/osmocom/gsm/gsup.h b/include/osmocom/gsm/gsup.h
index e246639..0ef5a75 100644
--- a/include/osmocom/gsm/gsup.h
+++ b/include/osmocom/gsm/gsup.h
@@ -154,6 +154,11 @@
 	OSMO_GSUP_SESSION_STATE_END		= 0x03,
 };
 
+extern const struct value_string osmo_gsup_session_state_names[];
+static inline const char *
+osmo_gsup_session_state_name(enum osmo_gsup_session_state val)
+{	return get_value_string(osmo_gsup_session_state_names, val); }
+
 /*! parsed/decoded PDP context information */
 struct osmo_gsup_pdp_info {
 	unsigned int			context_id;