[PATCH] fix `make headers_install'

Fix this:

make[3]: *** No rule to make target
`/mnt/md0/devel/linux-git/include/linux/version.h', needed by
`/mnt/md0/devel/linux-git-obj/usr/include/linux/version.h'.  Stop.
make[2]: *** [linux] Error 2
make[1]: *** [headers_install] Error 2

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