nixos/gnome-desktop: enable colord service

Fixes this (line wrapped):

  $ gnome-control-center
  [... click on the "Color" item ...]
  (gnome-control-center:3977): color-cc-panel-WARNING **: \
    The name org.freedesktop.ColorManager was not provided by any .service files

With this patch applied, the above warnings are not printed and the GUI
shows some devices that can be managed (my printer and display). Without
this patch the GUI is empty (non-functional).

(cups will also complain in the journal with a similar message when
doing print jobs, without this patch.)