cpantesters failure of GtkBuildable in subclass

[Kevin Ryde <user42 zip com au>]

I've had some cpantesters test failures from my Gtk2-Ex-WidgetBits
nonsense

  http://www.cpantesters.org/cpan/report/7e29e62e-3156-11e2-b1c8-a5d7a290f8f5

with something about adding Gtk2::Buildable to a subclass

  GLib-GObject-WARNING **: cannot add interface type `GtkBuildable' to
  type `Gtk2__Ex__ToolItem__CheckButton', since type
  `Gtk2__Ex__ToolItem__CheckButton' already conforms to interface at
  
/home/src/perl/repoperls/installed-perls/perl/v5.17.5-522-g10ffaf9/165a/lib/site_perl/5.17.6/x86_64-linux/Glib/Object/Subclass.pm
  line 233.

The offending bit of Gtk2::Ex::ToolItem::CheckButton is presumably

  package Gtk2::Ex::ToolItem::CheckButton;
  ...
  use Glib::Object::Subclass
    'Gtk2::ToolItem',
    interfaces => [
                   # Gtk2::Buildable new in Gtk 2.12, omit if not available
                   Gtk2::Widget->isa('Gtk2::Buildable') ? 'Gtk2::Buildable' : (),
                  ],
    signals => ...
    properties => ...;

But it seems to work for me.  Andreas Koenig suggests perhaps the hash
randomization of perl 5.17.x.  Does that sound likely?

  https://rt.cpan.org/Ticket/Display.html?id=81631