xen: add static initialization of steal_clock op to xen_time_ops

pv_time_ops might be overwritten with xen_time_ops after the
steal_clock operation has been initialized already. To prevent calling
a now uninitialized function pointer add the steal_clock static
initialization to xen_time_ops.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>