BUG: Fixes headless build

Fixes headless build on Linux.

Due to the VTK9 (fixed in vtk-master) bug with ProcessEvents we pulled our own vtkXRenderWindowInteractor (!556 (merged)) into imstk for Linux builds. However, vtkGenericRenderWindowInteractor should be used when building headless on Linux. Else it will include X11 which we don't want in headless.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• No new dependecies were added (or the new dependencies have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Ran the CodeFormatEnforcer task and verified the header is in all new files
• Added iMSTK examples for all new major features (if any)