selftests: Add and export a kernel uapi headers path

Kernel uapi headers can be present at different paths depending upon
how the build was invoked. It becomes impossible for the tests to
include the correct headers directory. Set and export KHDR_INCLUDES
variable to make it possible for sub make files to include the header
files.

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