Add qt as optional dependency of paraview

In some cases paraview cannot find qt version as the qt project is enabled after paraview.

E.g:

-- Enabling paraview as requested.
-- Enabling qt as requested.

In that case qt_version is not set and when configurating paraview I get this error:

CMake Error at CMake/paraviewQt.cmake:20 (message):
  Supported Qt versions are "Auto", "5", "6".  But `PARAVIEW_QT_VERSION` is
  set to "".
Call Stack (most recent call first):
  CMakeLists.txt:116 (include)