vty: Introduce cs7-instance-iu

Similar to what we already have in osmo-msc.

Change-Id: Iaa9e1d06dd0430848ef4f7498a3c15d13f899904
diff --git a/include/osmocom/sgsn/sgsn.h b/include/osmocom/sgsn/sgsn.h
index 6e4f5ca..78803c7 100644
--- a/include/osmocom/sgsn/sgsn.h
+++ b/include/osmocom/sgsn/sgsn.h
@@ -112,6 +112,7 @@
 #if BUILD_IU
 	struct {
 		enum ranap_nsap_addr_enc rab_assign_addr_enc;
+		uint32_t cs7_instance;
 	} iu;
 #endif