Properly depend on Qt5WebKitWidgets, not only Qt5WebKit

Summary:
The code currently depending on MARBLE_NO_WEBKIT actually
depends also on Qt5WebKitWidgets. And there are platforms
where Qt5WebKit exists, but not Qt5WebKitWidgets, so
the old check would fail.

Reviewers: rahn, nienhueser

Reviewed By: nienhueser

Differential Revision: https://phabricator.kde.org/D1430