Fix: Autoprefixer is now working (#16351)

The autoprefixer not working broke the phantomjs backend png rendering

Fixes #16345

scripts/webpack/postcss.config.js

@@ -1,7 +1,9 @@
-module.exports = {
-  plugins: {
-    'autoprefixer': {},
-    'postcss-reporter': {},
-    'postcss-browser-reporter': {},
-  }
-}
+module.exports = () => {
+  return {
+    plugins: {
+      autoprefixer: {},
+      'postcss-reporter': {},
+      'postcss-browser-reporter': {},
+    }
+  };
+};