lint_po: ignore unknown-message-flag errors

This error breaks CI with PO files generated by recent po4a.

I've asked i18nspector upstream to fix this: https://github.com/jwilk/i18nspector/issues/11

I've created an issue to revert this once this is fixed in a version of i18nspector that we can install everywhere relevant: tails#18988 (closed).