chore: fix type

Author: sapphi-red

packages/vite/src/node/plugins/css.ts

@@ -483,6 +483,7 @@ export function cssPostPlugin(config: ResolvedConfig): RolldownPlugin {
       return path.join(path.dirname(assetFileNames), cssAssetNameDir)
     } else {
       return path.dirname(
+        // @ts-expect-error rolldown does not support function `assetFileNames`
         assetFileNames({
           type: 'asset',
           name: cssAssetName,