selftests: stop using KSFT_KHDR_INSTALL

Stop using the KSFT_KHDR_INSTALL flag as installing the kernel headers
from the kselftest Makefile is causing some issues.  Instead, rely on
the headers to be installed directly by the top-level Makefile
"headers_install" make target prior to building kselftest.

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