xen/Makefile: fix dom-y build

We need to add $(dom0-y) to obj-$(CONFIG_XEN_DOM0) after dom0-y is
defined otherwise we end up adding nothing.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>