Export <linux/a.out.h> to userspace again.

This seems to have been removed accidentally in commit
ed7b1889

 ("Unexport asm/page.h"), but
wasn't supposed to have been -- the original patch at
http://lkml.org/lkml/2007/10/30/144 just moved it from $(header-y) to
$(unifdef-y)

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>