src: fix -Wextra warning in WriteFileSync

`req.result` is `ssize_t` while `UV_EIO` is an enumerator, which GCC
flags as mixing enumerated and non-enumerated types in a conditional
expression.

Assisted-by: Devin
Signed-off-by: Antoine du Hamel <duhamelantoine1995@gmail.com>

Author: aduh95

src/node_file_utils.cc

@@ -59,7 +59,7 @@ int WriteFileSync(const char* path, uv_buf_t* bufs, size_t buf_count) {
                 nullptr);
     if (req.result <= 0) {  // Error during write.
       // UV_EIO should not happen unless the file system is full.
-      int err = req.result < 0 ? req.result : UV_EIO;
+      int err = req.result < 0 ? static_cast<int>(req.result) : UV_EIO;
       uv_fs_req_cleanup(&req);
       uv_fs_close(nullptr, &req, fd, nullptr);
       uv_fs_req_cleanup(&req);