WebEngine: Fix include issue for PV based applications

Building a custom application against a ParaView with WebEngine enabled is now possible.

In CMake/paraview_client_main.cxx.in there is

#if PARAVIEW_USE_QTWEBENGINE
#include <QWebEngineUrlScheme>
#endif

However the Qt WebEngine component was not linked in the custom app because the variable PARAVIEW_USE_QTWEBENGINE was missing, making the conditional linking here https://gitlab.kitware.com/paraview/paraview/-/blob/master/CMake/ParaViewClient.cmake#L380 never evaluated.