Fix leak of 'pixbuf' gsb_gui_navigation_update_account_iter()
GObject-derived types are created with a reference count of 1 (unlike
GInitiallyUnowned-derived types which are created with a floating ref)
so they need to be unref()ed explicitly at some point.
Since the GtkTreeStore increments the reference count, we can unref() the
pixbuf after adding it to the store.
(cherry picked from commit fe6638ad9b82f0108a42b8253529b3ad4f8a68e3)