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/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index f98f7c9..4d9811b 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -469,6 +469,7 @@
 osmo_gsup_encode;
 osmo_gsup_decode;
 osmo_gsup_message_type_names;
+osmo_gsup_session_state_names;
 osmo_gsup_get_err_msg_type;
 
 osmo_oap_encode;