switch paging code to use a timer again, since not all BTS send reliable CCCH load indications

diff --git a/include/openbsc/gsm_data.h b/include/openbsc/gsm_data.h
index 25da171..7c320d8 100644
--- a/include/openbsc/gsm_data.h
+++ b/include/openbsc/gsm_data.h
@@ -253,6 +253,8 @@
 	struct gsm_paging_request *last_request;
 	struct gsm_bts *bts;
 
+	struct timer_list work_timer;
+
 	/* load */
 	u_int16_t available_slots;
 };