manifest: Add gnome-backgrounds
manifest: Add gnome-backgrounds

Per request of mclasen.

chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,GNOME/gnome-ostree,chergert/gnome-continuous,GNOME/gnome-ostree,chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,GNOME/gnome-ostree