sms: Make default SMS validity period configurable via VTY

Change-Id: Ie2c81c1d5795dd9aaf07b9766013f20b01abc762
diff --git a/include/osmocom/msc/sms_queue.h b/include/osmocom/msc/sms_queue.h
index ccc56ab..c3a6cfb 100644
--- a/include/osmocom/msc/sms_queue.h
+++ b/include/osmocom/msc/sms_queue.h
@@ -13,6 +13,7 @@
 	int max_pending;			/* maximum number of gsm_sms_pending in RAM */
 	bool delete_delivered;			/* delete delivered SMS from DB? */
 	bool delete_expired;			/* delete expired SMS from DB? */
+	unsigned int default_validity_mins;	/* default validity period in minutes */
 };
 
 struct sms_queue_config *sms_queue_cfg_alloc(void *ctx);