Avoid overwriting Qt5_FOUND with Qt5DBus or Qt5WebEngine disabled
Summary: Fixes at least a big cmake warning and potentially ppc64 build.
Test Plan: Builds fine with and without Qt5WebEngine.
Reviewers: Marble, rahn
Reviewed By: Marble, rahn
Subscribers: rahn, marble-devel, kde-edu
Differential Revision: https://phabricator.kde.org/D18141