BUILD/MINOR: fix build when USE_THREAD is not defined

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.