BUG/BUILD: threads: unbreak build without threads

The build without threads was once again broken.

This issue was introduced in commit ba86c6c ("MINOR: threads: Be sure to
remove threads from all_threads_mask on exit").

This is exactly the same problem as last time it happened, because of
all_threads_mask not being defined with USE_THREAD=

This must be backported in 1.8

src/haproxy.c

@@ -2473,9 +2473,8 @@ static void *run_thread_poll_loop(void *data)
 	list_for_each_entry(ptdf, &per_thread_deinit_list, list)
 		ptdf->fct();
 
-	HA_ATOMIC_AND(&all_threads_mask, ~tid_bit);
-
 #ifdef USE_THREAD
+	HA_ATOMIC_AND(&all_threads_mask, ~tid_bit);
 	if (tid > 0)
 		pthread_exit(NULL);
 #endif