BUILD/MINOR: fix build when USE_THREAD is not defined

src/queue.o: In function `pendconn_redistribute':
/home/ilia/haproxy/src/queue.c:272: undefined reference to `thread_want_sync'
src/queue.o: In function `pendconn_grab_from_px':
/home/ilia/haproxy/src/queue.c:311: undefined reference to `thread_want_sync'
src/queue.o: In function `process_srv_queue':
/home/ilia/haproxy/src/queue.c:184: undefined reference to `thread_want_sync'
collect2: error: ld returned 1 exit status
make: *** [Makefile:900: haproxy] Error 1

To be backported to 1.8.
diff --git a/src/queue.c b/src/queue.c
index 7cad813..1c730c7 100644
--- a/src/queue.c
+++ b/src/queue.c
@@ -181,7 +181,7 @@
 	HA_SPIN_UNLOCK(PROXY_LOCK,  &p->lock);
 
 	if (remote)
-		thread_want_sync();
+		THREAD_WANT_SYNC();
 }
 
 /* Adds the stream <strm> to the pending connection list of server <strm>->srv
@@ -269,8 +269,7 @@
 	HA_SPIN_UNLOCK(SERVER_LOCK, &s->lock);
 
 	if (remote)
-		thread_want_sync();
-
+		THREAD_WANT_SYNC();
 	return xferred;
 }
 
@@ -308,8 +307,7 @@
 	HA_SPIN_UNLOCK(PROXY_LOCK, &s->proxy->lock);
 
 	if (remote)
-		thread_want_sync();
-
+		THREAD_WANT_SYNC();
 	return xferred;
 }