Requiring the WebKitWidgets Qt5 Module

Summary:
This is required for QWebPage, see

http://doc.qt.io/qt-5/sourcebreaks.html#changes-to-qt-webkit

Test Plan: Build RKward :)

Reviewers: tfry

Reviewed By: tfry

Subscribers: meikm, Martchus, RKWard-Devel-list

Differential Revision: https://phabricator.kde.org/D8403