mncc: properly export osmo_mncc_name()

For some strange reason, the osmo_mncc_name() inline function
was not in the mncc.h header, but in the mncc.c file. Let's fix that.

Change-Id: I2c3666510c981dffa4ba25bed517fd7ebd1250f5
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 8c64020..c4b1b75 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -466,6 +466,7 @@
 osmo_msisdn_str_valid;
 
 osmo_mncc_stringify;
+osmo_mncc_names;
 _osmo_mncc_log;
 
 local: *;