kbuild: re-export VERSION, PATCHLEVEL, SUBLEVEL

Eric Sandeen <sandeen@sgi.com> pointed out that it is usefull to have
access to VERSION, PATCHLEVEL, SUBLEVEL in external modules, and gooling
a litte confirmed this.
So re-export them.
Usage within the kernel is still discouraged but possible.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>