example: add postcss as with-turbopack example devDep (#41795)

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## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Co-authored-by: JJ Kasper <jj@jjsweb.site>

examples/with-turbopack/package.json

@@ -32,6 +32,7 @@
     "autoprefixer": "^10.4.7",
     "eslint": "8.19.0",
     "eslint-config-next": "latest",
+    "postcss": "^8.4.14",
     "prettier": "^2.7.1",
     "prettier-plugin-tailwindcss": "^0.1.12",
     "tailwindcss": "^3.1.6",