Bugfix: Do not include irregular files in backup

Since restic 0.17.1, files with type `irregular` could incorrectly be included
in snapshots. This is most likely to occur when backing up special file types
on Windows that cannot be handled by restic.

This has been fixed.

When running the `check` command this bug resulted in an error like the
following:

```
  tree 12345678[...]: node "example.zip" with invalid type "irregular"
```

Repairing the affected snapshots requires upgrading to restic 0.17.2 and then
manually running `restic repair snapshots --forget`. This will remove the
`irregular` files from the snapshots.

https://github.com/restic/restic/pull/5057
https://forum.restic.net/t/errors-found-by-check-1-invalid-type-irregular-2-ciphertext-verification-failed/8447/2