Make sure we don't override QIcon::themeName when we should not

Summary:
KirigamiPlugin is lives in a separate thread than QApplication and themeName initialization is a race condition.
This way at least we limit the initialisation to non-linux desktop systems.

Test Plan: Now kalgebramobile and kirigamigallery show the icons on my system

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D17829