gsm: Introduce functions to convert between FN and RFN (Reduced FN)

Implementation ported from osmo-pcu.git
e98b315d12fb009359410809f4169f9380f3d933, function bts_rfn_to_fn().

This functionality can be used by osmo-pcu, libosmo-gprs or any other
related code which needs to handle RFNs.

Change-Id: Ib71e8da976f6cc84c3a4ab17b0a8c2101492e243
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 951cbcb..49db282 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -544,6 +544,7 @@
 osmo_dump_gsmtime;
 osmo_dump_gsmtime_buf;
 osmo_dump_gsmtime_c;
+gsm_rfn2fn;
 
 gsm_milenage;
 gsm_septet_encode;