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/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index dd4722e..31717d5 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -287,6 +287,7 @@
 gsm48_mm_msgtype_names;
 gsm48_cc_msgtype_names;
 gsm48_pdisc_msgtype_name;
+gsm48_reject_value_names;
 
 gsm_7bit_decode;
 gsm_7bit_decode_ussd;