win: fix mingw build error

`UV__UNUSED()` does not evaluate to nothing with MinGW, use something
else instead to squelch the unused argument warning.

PR-URL: https://github.com/libuv/libuv/pull/1882
Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/win/thread.c

@@ -374,7 +374,7 @@ int uv_cond_init(uv_cond_t* cond) {
 
 void uv_cond_destroy(uv_cond_t* cond) {
   /* nothing to do */
-  UV__UNUSED(cond);
+  (void) &cond;
 }