[gtkmm] sigc3: Use the newer notify callback signature.

[Murray Cumming <murrayc src gnome org>]

commit b7a17f826a15966b35a0d2c868d42ad18498795a
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Nov 11 13:49:43 2016 +0100

    sigc3: Use the newer notify callback signature.

 tests/builder/main.cc |    6 ++----
 1 files changed, 2 insertions(+), 4 deletions(-)
---
diff --git a/tests/builder/main.cc b/tests/builder/main.cc
index 65bb33c..961cfa2 100644
--- a/tests/builder/main.cc
+++ b/tests/builder/main.cc
@@ -76,16 +76,14 @@ const char gladefile[] =
   "</object>"
 "</interface>";
 
-void* on_managed_button_deleted(void* /* data */)
+void on_managed_button_deleted(sigc::notifiable* /* data */)
 {
   std::cout << "Gtk::Button in window deleted" << std::endl;
-  return nullptr;
 }
 
-void* on_orphaned_button_deleted(void* /* data */)
+void on_orphaned_button_deleted(sigc::notifiable* /* data */)
 {
   std::cout << "Orphaned Gtk::Button deleted" << std::endl;
-  return nullptr;
 }
 
 class DerivedButton : public Gtk::Button