kbuild, headers.sh: Don't make archheaders explicitly

We don't need to explicitly invoke the archheaders target because of
the dependency on __headers in the Makefile.

Reported-and-tested-by: Michal Marek <mmarek@suse.cz>
Link: http://lkml.kernel.org/r/4ECA8991.20302@suse.cz
Signed-off-by: H. Peter Anvin <hpa@zytor.com>