hplip: Add missing library

Otherwise, things like `hp-setup` fail finding gi:

```
HP Linux Imaging and Printing System (ver. 3.20.5)
Printer/Fax Setup Utility ver. 9.0

Copyright (c) 2001-18 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

Traceback (most recent call last):
  File "/nix/store/rmzq8c4n47s4q4p31w18waz0zrba2ljl-hplip-3.20.5/share/hplip/setup.py", line 314, in <module>
    ui = import_module(ui_package + ".setupdialog")
  File "/nix/store/n0b076p351ma864q38if4yglsg99hw2s-python3-3.8.3/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/nix/store/rmzq8c4n47s4q4p31w18waz0zrba2ljl-hplip-3.20.5/share/hplip/ui5/setupdialog.py", line 31, in <module>
    from base import device, utils, models, pkit
  File "/nix/store/rmzq8c4n47s4q4p31w18waz0zrba2ljl-hplip-3.20.5/share/hplip/base/pkit.py", line 38, in <module>
    from gi import _gobject as gobject
ModuleNotFoundError: No module named 'gi'
```