ASCI: Add callref to assignment command

The (optional) call reference is required to assign a calling subscriber
to a voice group/bcast channel. The BSC can then determine to which
existing VGCS/VBS channel the MS is assigned to.

This IE is part of the GSM standard TS 48.008 (see ยง3.2.1.1)

Change-Id: I7955c6e0eebc930f85f360dda46be17cbd39e181
Related: OS#4854
diff --git a/include/osmocom/msc/ran_msg.h b/include/osmocom/msc/ran_msg.h
index 0dadd4f..dc1483c 100644
--- a/include/osmocom/msc/ran_msg.h
+++ b/include/osmocom/msc/ran_msg.h
@@ -109,6 +109,8 @@
 	bool call_id_present;
 	uint32_t call_id;
 	struct osmo_lcls *lcls;
+	bool callref_present;
+	struct gsm0808_group_callref callref;
 };
 
 struct ran_cipher_mode_command {