kselftest/alsa: Ensure _GNU_SOURCE is defined

The pcmtest driver tests use the kselftest harness which requires that
_GNU_SOURCE is defined but nothing causes it to be defined.  Since the
KHDR_INCLUDES Makefile variable has had the required define added let's
use that, this should provide some futureproofing.

Fixes: daef47b8

 ("selftests: Compile kselftest headers with -D_GNU_SOURCE")
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>