gtk-demo: Plug another leak

We were leaking the builder in the css blendmodes demo,
by creating a ref cycle. This was showing up as
the list entry not going back to upright after
closing the window.

demos/gtk-demo/css_blendmodes.c

@@ -134,6 +134,8 @@ do_css_blendmodes (GtkWidget *do_widget)
                                                   GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
 
       setup_listbox (builder, provider);
+
+      g_object_unref (builder);
     }
 
   if (!gtk_widget_get_visible (window))