Added 'missing' code in mod/checklist/backup/moodle2/

...preventing restores from completing, as described in [MDL-34172](https://tracker.moodle.org/browse/MDL-34172).

Hi Davo. Am going through some course restore issues (as per [MDL-34172](https://tracker.moodle.org/browse/MDL-34172)) and [this comment](https://tracker.moodle.org/browse/MDL-34172?focusedCommentId=206139&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-206139) seems to define the problem and suggest a solution.  As the Checklist mod doesn't have the code suggested here (but allowing that I have seen no problems with restores specific to the Checklist mod) I have created this PR in the hope you find it useful.

~Paul.