kbuild: fix a.out.h export to userspace with O= build.

We need to check for existence of the a.out.h header in the source tree,
not the object tree, if we want it to get the right answer with O=.

Signed-off-by: David Woodhouse <david.woodhouse@intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>