xen: Export xen_reboot

The helper xen_reboot will be called by the EFI code in a later patch.

Note that the ARM version does not yet exist and will be added in a
later patch too.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Juergen Gross <jgross@suse.com>