add gsm48_reject_value_names

There seems to be quite some confusion / overlap between enum
gsm48_reject_value, gsm48_gsm_cause and gsm48_gmm_cause. I tried to go with
gsm48_gsm_cause_names[], but e.g. GSM48_REJECT_CONGESTION is not represented.

Instead of attempting to mix/merge those enums, provide a separate value string
array for enum gsm48_reject_value.

This will be used by osmo-msc's libvlr (refactoring of FSM result handling),
I27bf8d68737ff1f8dc6d11fb1eac3d391aab0cb1.

Change-Id: I6661f139e68a498fb1bef10c266c2f064b72774a
diff --git a/include/osmocom/gsm/protocol/gsm_04_08.h b/include/osmocom/gsm/protocol/gsm_04_08.h
index 4a821cc..6a5fb3b 100644
--- a/include/osmocom/gsm/protocol/gsm_04_08.h
+++ b/include/osmocom/gsm/protocol/gsm_04_08.h
@@ -1509,6 +1509,10 @@
 	GSM48_REJECT_MSC_TMP_NOT_REACHABLE	= 16,
 };
 
+extern const struct value_string gsm48_reject_value_names[];
+static inline const char *gsm48_reject_value_name(enum gsm48_reject_value val)
+{ return get_value_string(gsm48_reject_value_names, val); }
+
 enum chreq_type {
 	CHREQ_T_EMERG_CALL,
 	CHREQ_T_CALL_REEST_TCH_F,