community/gnome-feeds: add missing dependency

Running gfeeds without py3-lxml-html-clean installed prints:

raise ImportError(
ImportError: lxml.html.clean module is now a separate project lxml_html_clean.
Install lxml[html_clean] or lxml_html_clean directly.

NOTE: I also noticed, that abuild complains about __pycache__ files. I'm not sure, but maybe adding a pyc subpackage would help?