tests/unit: build pbkdf test on macOS

Add CONFIG_DARWIN to the pbkdf test build condition, since we have a way
to measure CPU time on this platform since commit bf98afc75efedf1.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
1 file changed