Add APN utility function to libosmogsm

The current functions are used to 'qualify' an APN from the
user-supplied APN name (name identifier) towards the fully-qualified
APN name which is used in the .grps DNS zone.
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index b7c5158..22428a2 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -265,5 +265,8 @@
 ipa_prepend_header_ext;
 ipa_send;
 
+osmo_apn_qualify;
+osmo_apn_qualify_from_imsi;
+
 local: *;
 };