When checking kernel headers, take linux/version.h from the build directory instead of the main headers.
When checking kernel headers, take linux/version.h from
the build directory instead of the main headers.

Fixes commit 2522 when build and headers directories
are split.

git-svn-id: 29c1264a5cf5e3532df57b06678e7347571c1a3c@2552 f1ba3bf5-cb5c-402b-92a9-7c6bdc83a356

ananos/open-mx,ananos/open-mx,ananos/xen2mx,ananos/xen2mx,ananos/xen2mx,ananos/open-mx,ananos/xen2mx