sms: Introduce a SMS queue that will be responsible for delivery

The SMSqueue will be responsible of sending to the user. It will
do so in a loop and will also try not to overload the BTS. This
means the throughput of SMS will be limited.
diff --git a/openbsc/include/openbsc/gsm_data.h b/openbsc/include/openbsc/gsm_data.h
index 5badde2..4c87f83 100644
--- a/openbsc/include/openbsc/gsm_data.h
+++ b/openbsc/include/openbsc/gsm_data.h
@@ -5,6 +5,7 @@
 
 struct osmo_msc_data;
 struct osmo_bsc_sccp_con;
+struct gsm_sms_queue;
 
 enum gsm_phys_chan_config {
 	GSM_PCHAN_NONE,
@@ -734,6 +735,7 @@
 
 	/* subscriber related features */
 	int keep_subscr;
+	struct gsm_sms_queue *sms_queue;
 };
 
 #define SMS_HDR_SIZE	128