gnome3.gnome-control-center: add grilo-plugins

Opening a background change dialogue produces the following errors in the console:

    (gnome-control-center:27270): Grilo-WARNING **: 21:57:21.349: [registry] ../src/grl-registry.c:1354: Could not open directory '/nix/store/q8wdbr08617p3xqxdg8935p31fha020f-grilo-0.3.9/lib/grilo-0.3': Error opening directory “/nix/store/q8wdbr08617p3xqxdg8935p31fha020f-grilo-0.3.9/lib/grilo-0.3”: No such file or directory
    (gnome-control-center:27270): Grilo-WARNING **: 21:57:21.352: [registry] ../src/grl-registry.c:1461: Plugin 'grl-flickr' not available
    (gnome-control-center:27270): background-cc-panel-WARNING **: 21:57:21.352: Plugin “grl-flickr” not available

Apparently, there are no plugins in `grilo` package – everything is in `grilo-plugins`, so we need to add `grilo-plugins` to inputs. The setup hook will make the plug-ins available through an environment variable:

https://gitlab.gnome.org/GNOME/grilo/blob/grilo-0.3.9/src/grilo.c#L111-135