RSL: add timer for lchan activation/deactivation without BTS response

The timer callback will simply reset the lchan state to NONE in order
to prevent channels getting stuck in 'activation requested' or
'deactivation requested' states.
diff --git a/openbsc/include/openbsc/gsm_data_shared.h b/openbsc/include/openbsc/gsm_data_shared.h
index e3ab5f4..89375cf 100644
--- a/openbsc/include/openbsc/gsm_data_shared.h
+++ b/openbsc/include/openbsc/gsm_data_shared.h
@@ -182,6 +182,7 @@
 	struct osmo_timer_list T3101;
 	struct osmo_timer_list T3111;
 	struct osmo_timer_list error_timer;
+	struct osmo_timer_list act_timer;
 
 	/* table of neighbor cell measurements */
 	struct neigh_meas_proc neigh_meas[MAX_NEIGH_MEAS];