windows: Add a common Windows header for platform specific wrappers

Include some more basic system headers in utils.h, so we can use that common
header on the different platforms.

src/libstrongswan/crypto/crypto_tester.c

@@ -14,8 +14,10 @@
  * for more details.
  */
 
-#define _GNU_SOURCE
-#include <dlfcn.h>
+#ifdef HAVE_DLADDR
+# define _GNU_SOURCE
+# include <dlfcn.h>
+#endif
 #include <time.h>
 
 #include "crypto_tester.h"