gpodder: Update to 3.8.0

I bumped the version number and since gpodder is a GTK app, I added the path to the gnome standard icon theme.

Best,
Sven
Are you sure your change does anything at all? XDG_ICON_DIRS is empty there without putting icon themes in the build inputs. Look at how other gnome apps use hicolor icon theme and gnome icon theme (symbolic).