gdk-pixbuf-io: add some missing allow-none annotations
gdk-pixbuf-io: add some missing allow-none annotations

For GCancellable arguments.

https://bugzilla.gnome.org/show_bug.cgi?id=683064

GNOME/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,Distrotech/gdk-pixbuf,GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf,GNOME/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,Distrotech/gdk-pixbuf