x86/xen/time: fix section mismatch for xen_init_time_ops()

The header declares this function as __init but is defined in __ref
section.

Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>