[PATCH] headers_check: use a different default directory

`make headers_check' wants to go and write stuff in /lib/modules, which
requires root, whic is unfortunate.

In fact, there's no _particular_ reason for headers_install to put it there
either -- it can go into a subdirectory of the build tree in both cases.
It's not intended to go directly into /usr/include, which is why we didn't
put it there -- and we certainly don't want people screwing around with
symlinking to it.  It's for distributors to take away and do stuff with, so
leaving it in $(objtree) is fine, even in the headers_install case.

I picked $(objtree)/usr/include but I have no _particular_ preference
for that; it just seemed reasonable.

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