Fix omitted lazy import for xlm-prophetnet
# What does this PR do?

Fixes import code for the xlm-prophetnet model to support lazy import. Continued from #13015.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?

The failures are unrelated to this PR (and the underlying problem is fixed on master) so can be safely ignored.<|||||>One last thing: can you run `make style` on your branch to fix the quality issue? We should be good to go after.