[vtkusers] Missing install rule for QVTKInteractor.h

[Ben Medina]

Hi all,

I'm building the latest VTK, and my installs are missing the
QVTKInteractor.h header. This header is #included by QVTKWidget.h, so
it needs to be in the installation. I can work around this for now by
manually copying the file over, but I'm sure others will run into this
soon.

Also, it seems that QVTKWidget.h could get by with forward declaring
QVTKInteractor, rather than a #include.

Thanks,
Ben