[PATCH] Add CONFIG_HEADERS_CHECK option to automatically run 'make headers_check'

In order to encourage people to notice when they break the exported
headers, add a config option which automatically runs the sanity checks
when building vmlinux.  That way, those who use allyesconfig will notice
failures.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>