Set walk_dirs to false in permastruct, fix #14

This should fix a conflict when a page has the same slug has a content type and the content type has `has_archive => false`.

I've quickly tested it with different polylang and content type settings and it seems to work in all cases. Eventually some more tests would be nice before merging this.

See https://github.com/KLicheR/wp-polylang-translate-rewrite-slugs/issues/14 for more informations.