haproxy: put back dependency on __sync_*_4

haproxy: put back dependency on __sync_*_4

__sync_*_4 intrinsics is used by common/hathreads.h when gcc < 4.7 but
also by contrib/tcploop.c or include/proto/shctx.h so put back a
mandatory dependency on BR2_TOOLCHAIN_HAS_SYNC_4 which has been
wrongly removed by commit f22fca4bf27f884950dd7804020e5d5193b570b9
("haproxy: tweak thread activation").

Fixes:
 - http://autobuild.buildroot.net/results/52c88f2f27e1a3c42cf71554cf680da86d933ef6

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>