Explicitly type ColorUtils lambda

Summary:
Fixes Qt 5.12 CI failure:

no known conversion from ColorUtils* (*)(QQmlEngine*, QJSEngine*) to QObject* (*)(QQmlEngine*, QJSEngine*)

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D28665