kbuild: make sure autoksyms.h exists early

Some people are able to trigger a race where autoksyms.h is used before
its empty version is even created.  Let's create it at the same time as
the directory holding it is created.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Prarit Bhargava <prarit@redhat.com>
Tested-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>