[gtkmm] demos: Use the newer libsigc++ syntax.

[Murray Cumming <murrayc src gnome org>]

commit 21d410e5357ac9a671867de25e1f1a050e286ca8
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Nov 11 13:50:01 2016 +0100

    demos: Use the newer libsigc++ syntax.

 demos/gtk-demo/demos.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/demos/gtk-demo/demos.h b/demos/gtk-demo/demos.h
index 8a95283..f18c879 100644
--- a/demos/gtk-demo/demos.h
+++ b/demos/gtk-demo/demos.h
@@ -3,7 +3,7 @@
 
 #include "demowindow.h"
 
-typedef sigc::slot<Gtk::Window*> type_slotDo;
+typedef sigc::slot<Gtk::Window*()> type_slotDo;
 
 struct Demo
 {