Always export VTK-m functor symbols
This is done to avoid warnings when compiling VTK-m consumers with different defaults for symbol visiblity. AKA avoid warnings like:
warning: ‘vtkm::worklet::WorkletMapField’ declared with greater visibility than the type of its field ‘vtkm::worklet::WorkletMapField::<anonymous>’ [-Wattributes]
class WorkletMapField : public vtkm::worklet::internal::WorkletBase
^~~~~~~~~~~~~~~