selftests: use "$(MAKE)" instead of "make" for headers_install

If top make invocation uses -j4 or larger, this patch reduces
"make headers_install" subtask run time from 30 to 7 seconds.

CC: Shuah Khan <shuah@kernel.org>
CC: Shuah Khan <skhan@linuxfoundation.org>
CC: linux-kselftest@vger.kernel.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>