Install scalarbar internal header
Reported here: https://discourse.vtk.org/t/vtk9-0-rc1-vtkscalarbaractorinternal-h-not-installed/2855
Cc: @tjcorona
Backport: release
Reported here: https://discourse.vtk.org/t/vtk9-0-rc1-vtkscalarbaractorinternal-h-not-installed/2855
Cc: @tjcorona
Backport: release