xen/preempt: use need_resched() instead of should_resched()

This code is used only when CONFIG_PREEMPT=n and only in non-atomic
context: xen_in_preemptible_hcall is set only in
privcmd_ioctl_hypercall().  Thus preempt_count is zero and
should_resched() is equal to need_resched().

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>