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 bf98afc75e.

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

tests/unit/test-crypto-pbkdf.c

@@ -25,7 +25,7 @@
 #include <sys/resource.h>
 #endif
 
-#if defined(_WIN32) || defined(RUSAGE_THREAD)
+#if defined(_WIN32) || defined(RUSAGE_THREAD) || defined(CONFIG_DARWNI)
 #include "crypto/pbkdf.h"
 
 typedef struct QCryptoPbkdfTestData QCryptoPbkdfTestData;