selftests: futex: Add the uapi headers include variable

Out of tree build of this test fails if relative path of the output
directory is specified. KBUILD_OUTPUT also doesn't point to the correct
directory when relative path is used. Thus out of tree builds fail.
Remove the un-needed include paths and use KHDR_INCLUDES to correctly
reach the headers.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>