ASCI: Add simple implementation of Group Call Register

This is a built-in data structure to store and handle voice group calls.

The GCR will be used by VGCS/VBS call control.
 (Chg-Id: I9947403fde8212b66758104443c60aaacc8b1e7b)

The GCR will be used by VTY code.
 (Chg-Id: I5bd034a62fc8b483f550d29103c2f7587198f590)

Change-Id: Ia74a4a865f943c5fb388cd28f9406005c92e663e
Related: OS#4854
diff --git a/include/osmocom/msc/gsm_data.h b/include/osmocom/msc/gsm_data.h
index 2be4c35..6c0dac9 100644
--- a/include/osmocom/msc/gsm_data.h
+++ b/include/osmocom/msc/gsm_data.h
@@ -269,6 +269,11 @@
 
 	/* SMS queue config parameters */
 	struct sms_queue_config *sms_queue_cfg;
+
+	/* ASCI feature support */
+	struct {
+		struct llist_head gcr_lists;
+	} asci;
 };
 
 struct smpp_esme;