xfce4-settings 4.8.2: change default icon theme to 'gnome'

'task-xfce4-base' doesn't install the 'Rodent' theme leading to missing icons. With this patch all icons show up.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Acked-by: Andreas Müller <schnitzeltony@gmx.de>