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