xen: XEN_ACPI_PROCESSOR is Dom0-only

[ Upstream commit c4f9d9cb2c29ff04c6b4bb09b72802d8aedfc7cb ]

Add a respective dependency.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>