compel_syscall(): make ret argument signed

I saw this line in the code

	unsigned long sret = -ENOSYS;

and ended up with this patch. Note syscall(2) man page says return value
is long -- who am I to disagree?

travis-ci: success for More polishing for compel cli
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>