MNCC: Add MNCC to string dumper

As MNCC is rather hard to debug (wireshark cannot trace UNIX domain
sockets), let's add our own decoder that we can use from related
debug log statements in the respective programs.

Change-Id: I216aaf70868ba5f3860a60c4b2442957531a3011
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 985ec83..9a7073d 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -427,5 +427,8 @@
 osmo_imsi_str_valid;
 osmo_msisdn_str_valid;
 
+osmo_mncc_stringify;
+_osmo_mncc_log;
+
 local: *;
 };