Fix labkit backwords_compatibility build

What does this MR do?

Related to: labkit#44 (closed)

Commands ran (locally):

go mod edit -replace gitlab.com/gitlab-org/labkit=../labkit && go mod tidy -compat=1.17 && go mod edit -dropreplace gitlab.com/gitlab-org/labkit

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines