xfce4-settings: recommend adwaita, not gnome-icon-theme

RRECOMMEND adwaita-icon-theme instead of gnome-icon-theme (as latter
is removed from oe-core).

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>