sms: Make the max_pending tunable via a vty runtime setting

Make it possible to change the max pending via the vty. This
can be useful to play with the performance of the queue.
diff --git a/openbsc/include/openbsc/sms_queue.h b/openbsc/include/openbsc/sms_queue.h
index 5bfa4b4..02dec73 100644
--- a/openbsc/include/openbsc/sms_queue.h
+++ b/openbsc/include/openbsc/sms_queue.h
@@ -10,5 +10,6 @@
 
 /* vty helper functions */
 int sms_queue_stats(struct gsm_sms_queue *, struct vty* vty);
+int sms_queue_set_max_pending(struct gsm_sms_queue *, int max);
 
 #endif