Plug memory leak in GtkWindow

Plug memory leak in GtkWindow

1,936 (112 direct, 1,824 indirect) bytes in 4 blocks are definitely lost in loss record 13,453 of 13,673
    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
    by 0x69615A1: cairo_region_create (cairo-region.c:196)
    by 0x683BFBF: gdk_cairo_region_create_from_surface (gdkcairo.c:455)
    by 0x670C882: set_grip_shape (gtkwindow.c:5020)
    by 0x670CFBF: resize_grip_create_window (gtkwindow.c:5271)
    by 0x670C466: gtk_window_realize (gtkwindow.c:4902)

Signed-off-by: Benjamin Otte <otte@redhat.com>