xen: Export xen_reboot

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>
OraBug: 26662731

(cherry picked from commit 5d9404e1185de8d508cd042761306495f727d7eb)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Conflicts:
	arch/x86/xen/xen-ops.h
	arch/x86/xen/enlighten.c
	(No changes to xen-ops.h, make xen_reboot() non-static)