[vtkusers] Using vtkPlyWriter

[Gonzalo Carmona Martinez]

Hello.

I´d like to use the VTK classes vtkPlyWriter and vtkPlyReader, but
the compiler doesn´t know the .h files vtkPlyWriter.h and
vtkPlyReader.h

I don´t know if I have to set any option in the CMakeLists.txt file.

Thanks for your help!