make sure allTimezones is set

Summary:
on Qt 5.6/5.7 changed signal was emitted at startup as well,
triggering plasmoid.configuration.onSelectedTimeZonesChanged
in Qt 5.8 this doesn't happen, giving an empty clock at startup
this makes sure allTimezones gets built

Test Plan: with Qt 5.8 the clock is shown correctly

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2946