Fix ia64 compilation IS_ERR and PTE_ERR errors.

When building ia64 kernel with CONFIG_XEN_SYS_HYPERVISOR, compiler reports
errors:

drivers/xen/sys-hypervisor.c: In function ‘uuid_show’:
drivers/xen/sys-hypervisor.c:125: error: implicit declaration of function ‘IS_ERR’
drivers/xen/sys-hypervisor.c:126: error: implicit declaration of function ‘PTR_ERR’

This patch fixes the errors.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Acked-by: Isaku Yamahata <yamahata@valinux.co.jp>