selftests: futex: set DEFAULT_INSTALL_HDR_PATH

If only futex selftest is compiled, uapi header files are copied to the
selftests/futex/functional directory. This copy isn't needed. Set the
DEFAULT_INSTALL_HDR_PATH variable to 1 to use the default header install
path only. This removes extra copy of header file.

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