chore: update example to use local postcss

Author: phated

examples/html-kitchen-sink/.storybook/main.js

@@ -12,7 +12,13 @@ module.exports = {
     '@storybook/addon-jest',
     '@storybook/addon-knobs',
     '@storybook/addon-links',
-    '@storybook/addon-postcss',
+    {
+      name: '@storybook/addon-postcss',
+      options: {
+        // eslint-disable-next-line global-require
+        postcss: require('postcss'),
+      },
+    },
     '@storybook/addon-storysource',
     '@storybook/addon-viewport',
   ],