selftests/landlock: fix broken include of linux/landlock.h

Revert part of the earlier changes to fix the kselftest build when
using a sub-directory from the top of the tree as this broke the
landlock test build as a side-effect when building with "make -C
tools/testing/selftests/landlock".

Reported-by: Mickaël Salaün <mic@digikod.net>
Fixes: a917dd94 ("selftests/landlock: drop deprecated headers dependency")
Fixes: f2745dc0 ("selftests: stop using KSFT_KHDR_INSTALL")
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>