gnu: zathura-{cb,ps,djvu,pdf-poppler}: Fix installation of desktop files.

Based on a patch by Alex Kost <alezost@gmail.com>.

* gnu/packages/pdf.scm (zathura-cb, zathura-ps, zathura-djvu)
  (zathura-pdf-poppler)[arguments]: In make-flags, set PREFIX instead of
  DESTDIR and adjust PLUGINDIR accordingly.