xen: fix header export to userspace

xen: fix header export to userspace

scripts/headers_install.pl prevents "__user" from being exported
to userspace headers, so just use compiler.h to make sure that
__user is defined and avoid the error.

unifdef: linux-next-20101112/xx64/usr/include/xen/privcmd.h.tmp: 79: Premature EOF (#if line 33 depth 1)

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc:	Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc:	xen-devel@lists.xensource.com (moderated for non-subscribers)
Cc:	virtualization@lists.osdl.org
Cc:	Tony Finch <dot@dotat.at>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>