xen: install xen/gntdev.h and xen/gntalloc.h

commit 564eb714 upstream.

xen/gntdev.h and xen/gntalloc.h both provide userspace ABIs so they
should be installed.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[bwh: Backported to 3.2: no renaming is required]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

include/xen/Kbuild

 header-y += evtchn.h
+header-y += gntalloc.h
+header-y += gntdev.h
 header-y += privcmd.h