xen: Fix up build warning with xen_init_time_ops() reference

Somehow the xen_init_time_ops() reference got out of sync in the 4.14.y
tree (or it never was in sync), and now there's a build warning.  Fix
that up by making xen_init_time_ops() be __init, not __ref.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>