windows: fix uv_backend_fd() return value

uv__new_artificial_error() returns a uv_err_t, not an int, and we can't use
uv__set_artificial_error() because the uv_loop_t argument is const.

src/win/core.c

@@ -172,7 +172,7 @@ void uv_loop_delete(uv_loop_t* loop) {
 
 
 int uv_backend_fd(const uv_loop_t* loop) {
-  return uv__new_artificial_error(UV_ENOSYS);
+  return -1;
 }