gdk3: don't use "Gdk::" in loader
gdk3: don't use "Gdk::" in loader

Because "Gdk::" is not constant value in the loader. It is the
variable named @base_module in the loader.

kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2