LCLS, TS 48.008: add GCR IE encoding/decoding

* add functions to encode Global Call. Ref. from TS 29.205 as 3GPP TS
  48.008 ยง3.2.2.115 information element
* add corresponding tests

Change-Id: I82ce0207dc8de50689a8806c6471ad7fbae6219d
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index e85ed6d..94ae76a 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -219,6 +219,8 @@
 gsm0808_lcls_config_names;
 gsm0808_lcls_control_names;
 gsm0808_lcls_status_names;
+gsm0808_enc_gcr;
+gsm0808_dec_gcr;
 
 gsm29118_msgb_alloc;
 gsm29118_create_alert_req;