replace gsm48_cc_msg_names[] with gsm48_cc_msg_name()

and implement the backend using value_string
diff --git a/include/osmocore/gsm48.h b/include/osmocore/gsm48.h
index 787cdd0..1e96357 100644
--- a/include/osmocore/gsm48.h
+++ b/include/osmocore/gsm48.h
@@ -5,8 +5,8 @@
 #include <osmocore/gsm48_ie.h>
 
 extern const struct tlv_definition gsm48_att_tlvdef;
-extern const char *cc_state_names[32];
-extern const char *gsm48_cc_msg_names[0x40];
+const char *gsm48_cc_state_name(uint8_t state);
+const char *gsm48_cc_msg_name(uint8_t msgtype);
 const char *rr_cause_name(uint8_t cause);
 
 void gsm48_generate_lai(struct gsm48_loc_area_id *lai48, uint16_t mcc,