selftests/landlock: drop deprecated headers dependency

The khdr make target has been removed, so drop it from the landlock
Makefile dependencies as well as related include paths that are
standard for headers in the kernel tree.

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Reported-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>