windows: Include <sys/stat.h> explicitly before overloading memset()/memcpy()

fstat() in newer MinGWs is defined as non-static inline. With our new static
inline memset()/memcpy() overloads, this raises a warning. To avoid it,
explicitly include <sys/stat.h> once before defining these overloads.

src/libstrongswan/utils/windows.h

@@ -26,6 +26,7 @@
 #include <direct.h>
 #include <inttypes.h>
 #include <unistd.h>
+#include <sys/stat.h>
 
 /* undef Windows variants evaluating values more than once */
 #undef min