main/postmarketos-base-ui: depend on tzdata

Most UIs already implicitly depend on this in some way, and from the user-facing point of view, it is sensible to have the full DB of timezones available, in case the user wants to swap timezone or requires working with them in different ways. Having it as a explicit dependency makes more sense than relying on backward dependencies.

Background: #2168 (closed) (comment 1426261355)