Prefer "require_once" over "require" in a few spots

[JJJ]

Is your enhancement related to a problem? Please describe.

We're manually including a file from the Two-Factor plugin in my WP User Profiles plugin, and it just-so-happens to be one of the few that are not require_onceed.

Proposed Solution

require_once is already predominantly used here, so it won't hurt to be consistent and match-up the stragglers.

Designs

None

Describe alternatives you've considered

We tried to work-around manually including Two-Factor files, but due to the amount of CSS & JS that is included and it being under-construction, it doesn't seem right to package copies of it all inside the WP User Profiles plugin (just to work around a notice).

Please confirm that you have searched existing issues in this repository.

Yes