LCLS, TS 29.205: add GCR routines

Add functions to encode and decode Global Call Reference as per
3GPP TS 29.205 Table B 2.1.9.1 add corresponding tests.

Change-Id: Iee95aa4e5c056645b6cb5667e4a067097d52dfbf
Related: OS#2487
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index dc4e0a6..e85ed6d 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -235,6 +235,9 @@
 gsm29118_create_release_req;
 gsm29118_create_service_abort_req;
 
+osmo_enc_gcr;
+osmo_dec_gcr;
+
 gsm0858_rsl_ul_meas_enc;
 
 gsm338_get_sms_alphabet;