Fix git builds against Qt 5.14
Summary:
It's not really acceptable to just error on everything since some of the
solutions (like in this case) might not be retrocompatible.
The error was:
kdecoration/src/decoration.cpp:386:37: error: no member named 'posF' in 'QWheelEvent'
Test Plan: Builds now
Reviewers: Frameworks, vkrause
Reviewed By: vkrause
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25671