[dashboards] Warning on configuration with Xcursor

on paraview-branch-master-[fedora31_shared_icc_mpi_python]

CMake Warning at VTK/Rendering/OpenGL2/CMakeLists.txt:359 (message):
  X11::Xcursor not found; custom cursors will be ignored.

Xcursor should either be added to the docker image or the warning should be a message.

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