Add :WINDOWS clause to CFFI:DEFINE-FOREIGN-LIBRARY.

Add :WINDOWS clause to CFFI:DEFINE-FOREIGN-LIBRARY.

```
 (cffi:define-foreign-library libcuda
+   (:windows "nvcuda.dll")
    (:darwin (:framework "CUDA"))
    (:unix (:or "libcuda.so" "libcuda64.so")))
```
