[goocanvasmm] Adapt dynamic cast for Glib::RefPtr as std::shared_ptr.

[Murray Cumming <murrayc src gnome org>]

commit 76f937fda3d3fad7952a3f166fe3038a90e6346f
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 7 15:03:34 2017 +0200

    Adapt dynamic cast for Glib::RefPtr as std::shared_ptr.

 examples/moving_shapes/window.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/examples/moving_shapes/window.cc b/examples/moving_shapes/window.cc
index d84ad65..217f689 100644
--- a/examples/moving_shapes/window.cc
+++ b/examples/moving_shapes/window.cc
@@ -73,7 +73,7 @@ Window::Window()
 void
 Window::on_item_created(const Glib::RefPtr<Goocanvas::Item>& item, const Glib::RefPtr<Goocanvas::ItemModel>& 
/* model */)
 {
-  auto group = Glib::RefPtr<Goocanvas::Group>::cast_dynamic(item);
+  auto group = std::dynamic_pointer_cast<Goocanvas::Group>(item);
   if(group)
     return ;