Try to fix build with MARBLE_NO_WEBKITWIDGETS
Summary: It is used in sources previously included above the definition.
Test Plan: Builds fine with and without Qt5WebEngineWidgets
Reviewers: Marble, rahn
Reviewed By: Marble, rahn
Subscribers: marble-devel, kde-edu
Differential Revision: https://phabricator.kde.org/D18143