Makefile: replace headers_install with headers for kselftest

Replace headers_install with headers as kselftest uses the header
files from within the kernel tree rather than from a system-wide
installation.

We can still run this directly:

  $ make O=build kselftest-all

and when building from the selftests directory:

  $ make O=build headers
  $ make O=build -C tools/testing/selftests all

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Reported-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>