MINOR: threads: Use __decl_hathreads instead of #ifdef/#endif

A #ifdef/#endif on USE_THREAD was added in the commit 0048dd04 ("MINOR: threads:
Fix build when we're not compiling with threads.") to conditionally define the
start_lock variable, because HA_SPINLOCK_T is only defined when HAProxy is
compiled with threads.

If fact, to do that, we should use the macro __decl_hathreads instead.

If commit 0048dd04 is backported in 1.8, this one can also be backported.
diff --git a/src/haproxy.c b/src/haproxy.c
index 8a4e8a5..34e8f75 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2422,9 +2422,7 @@
 {
 	struct per_thread_init_fct   *ptif;
 	struct per_thread_deinit_fct *ptdf;
-#ifdef USE_THREAD
-	static HA_SPINLOCK_T start_lock;
-#endif
+	__decl_hathreads(static HA_SPINLOCK_T start_lock);
 
 	tid     = *((unsigned int *)data);
 	tid_bit = (1UL << tid);