ia64/xen: fix the link error.

This patch fixes the following link error with xen_domu_defconfig.
Depending on compiler version, it doesn't link as follows.
So remove const and use __initdata for xen_iosapic_ops.

> arch/ia64/xen/xen_pv_ops.c:878: error: xen_iosapic_ops causes a section type conflict

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>