g_warning () is for environment errors. Additional tags than the
ones we require are not necessarily an error.

This behavior causes g-c-c to spam the journal with entries such
as:

Unknown Tag in /usr/share/gnome-background-properties/fedora-workstation-backgrounds.xml: license
Unknown Tag in /usr/share/gnome-background-properties/fedora-workstation-backgrounds.xml: email
Unknown Tag in /usr/share/gnome-background-properties/fedora-workstation-backgrounds.xml: author

Where "license", "email", and "author" are valid tags that we just
don't use.