[IA64] xen: use ARRAY_SIZE macro in xen_pv_ops.c

Replace sizeof(xen_branch_target) / sizeof(xen_branch_target[0]) with
ARRAY_SIZE(xen_branch_target) in arch/ia64/xen/xen_pv_ops.c

Signed-off-by: Nikitas Angelinas <nikitasangelinas@gmail.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>