gutenprint: wrap sbin/cups-genppd.5.2

This is used to generate ppd files for cups:

  $ cups-genppd.5.2 -M | grep p50
  canon-cp500
  escp2-p50
  $ cups-genppd.5.2 -p . escp2-p50

This will create the relevant ppd file in the current directory. This
can then be used by cups after setting services.drivers.drivers =
[pkgs.gutenprint]; and configuring the printer to use this ppd e.g.
via the web interface on http://localhost:631