firstboot: fix gtk warning about non-zero page size
    
    Pygtk documentation says the page size field is not used, but actually
    there will be a gtk warning below:
    
    /usr/share/firstboot/modules/firstboot_kdump.py:216: GtkWarning: GtkSpinButton: setting an adjustment with non-zero page size is deprecated
      self.kdumpMemspin = gtk.SpinButton(self.memAdjustment, 0, 0)
    
    Fix it by setting the last field to 0.
    
    Signed-off-by: Dave Young <dyoung@redhat.com>
    Reviewed-by: Vivek Goyal <vgoyal@redhat.com>