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/include/osmocom/gsm/gsm0808_utils.h b/include/osmocom/gsm/gsm0808_utils.h
index 90ff677..5d5803b 100644
--- a/include/osmocom/gsm/gsm0808_utils.h
+++ b/include/osmocom/gsm/gsm0808_utils.h
@@ -62,7 +62,7 @@
 struct osmo_lcls {
 	enum gsm0808_lcls_config config;   /**< §3.2.2.116 Configuration */
 	enum gsm0808_lcls_control control; /**< §3.2.2.117 Connection Status Control */
-	struct gsm29205_gcr *gcr;          /**< §3.2.2.115 Global Call Reference */
+	struct osmo_gcr_parsed *gcr;       /**< §3.2.2.115 Global Call Reference */
 	bool corr_needed;                  /**< §3.2.2.118 Correlation-Not-Needed */
 };