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

Tags: #marble, #kde_edu

Differential Revision: https://phabricator.kde.org/D18143