BUILD: listener: fix build warning on global_listener_rwlock without threads

The global_listener_rwlock was introduced by recent commit 13e86d947
("BUG/MEDIUM: listener: Fix race condition when updating the global mngmt
task"), but it's declared static and is not used when threads are disabled,
thus causing a warning to be emitted in this case. Let's just condition it
to thread usage to shut the warning.

This will need to be backported where the patch above is backported.

(cherry picked from commit 469fa479501f4807d9983ca46618aba3c4ec8cb7)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 98407862323632c16511bcf90f281eb5c4aa41c3)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 445b46d7d34f209e19da5334028fca16f875e95e)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/listener.c b/src/listener.c
index 007c686..4a38ab5 100644
--- a/src/listener.c
+++ b/src/listener.c
@@ -45,7 +45,7 @@
 /* list of the temporarily limited listeners because of lack of resource */
 static struct mt_list global_listener_queue = MT_LIST_HEAD_INIT(global_listener_queue);
 static struct task *global_listener_queue_task;
-static HA_RWLOCK_T global_listener_rwlock;
+__decl_thread(static HA_RWLOCK_T global_listener_rwlock);
 
 /* listener status for stats */
 const char* li_status_st[LI_STATE_COUNT] = {