Hotfix for fatal error on WP 5.7 due to private _builtin property

[andronocean]

This makes the WP_Statuses_Core_Status::_builtin property public, so it is accessible by is_post_status_viewable() (new in WP 5.7). This resolves fatal errors that made the post list inaccessible if the plugin was active, fixing #51.

[imath]

Thanks 👌