ASCI: Add option to switch on or off ASCI support

Switching ASCI support is controled via VTY. This added in a later
patch. (Chg-Id: I5bd034a62fc8b483f550d29103c2f7587198f590)

Change-Id: Id68deb69f7395f0f8f50b3820e9d51052a34f753
Related: OS#4854
diff --git a/include/osmocom/msc/gsm_data.h b/include/osmocom/msc/gsm_data.h
index 6c0dac9..119f093 100644
--- a/include/osmocom/msc/gsm_data.h
+++ b/include/osmocom/msc/gsm_data.h
@@ -272,6 +272,7 @@
 
 	/* ASCI feature support */
 	struct {
+		bool enable;
 		struct llist_head gcr_lists;
 	} asci;
 };