Replace ulong by unsigned long in vtkXRenderWindowInteractor.cxx

Hi, one more:

in Rendering/UI/vtkXRenderWindowInteractor.cxx l. 707 ff we have

    char* data = nullptr;
      Atom actualType;
      int actualFormat;
      ulong itemCount, bytesAfter;
      XGetWindowProperty(this->DisplayId, event->xselection.requestor, event->xselection.property,
        0, LONG_MAX, False, event->xselection.target, &actualType, &actualFormat, &itemCount,
        &bytesAfter, (unsigned char**)&data);

If ulong is replaced here by unsigned long we can compile on FreeBSD. I know, it is probaly not supported, but anyway this change appears to be closer to the standard. It seems to be the only place...