Fix broken deps since 2.20
    
    PDL::Graphics::PGPLOT is no longer provided but
    PDL::Demos::PGPLOT_demo still requires it.
    
    Now on F32:
    $ sudo dnf install /usr/bin/perldl
    Error:
     Problem: conflicting requests
      - nothing provides perl(PDL::Graphics::PGPLOT) needed by perl-PDL-2.20.0-2.fc32.x86_64
      - nothing provides perl(PDL::Graphics::PGPLOT::Window) needed by perl-PDL-2.20.0-2.fc32.x86_64
    
    Since 2.20, PDL will not install PDL::Graphics::PGPLOT if the true
    PGPLOT module is not installed.