kbuild: fix make headers_check with make 3.80

Commit 10b63956

 ("UAPI: Plumb the UAPI Kbuilds into the user header
installation and checking") introduced a dependency of make 3.81
due to use of $(or ...)

We do not want to lift the requirement to gmake 3.81 just yet...
Included are a straightforward conversion to $(if ...)

Bisected-and-tested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: <stable@vger.kernel.org> [v3.7+]
Signed-off-by: Michal Marek <mmarek@suse.cz>