Consider using require_once in the masterbar module

Does it matter if this is `require` or `require_once`? Jetpack seems to support `__DIR__ `, in case that is something you want to change.

_Originally posted by @obenland in https://github.com/Automattic/jetpack/pull/17762#discussion_r520677542_

Locate the masterbar module and inspect the require call mentioned in the issue. Check how nearby Jetpack modules handle repeated loading and __DIR__, then verify the module’s existing tests or loading path. Done means the loading approach is consistent and the masterbar still loads without duplicate-inclusion problems.

Written by the indexing model from the issue text.