gtk3: fix a symbol for Gdk::Window::WindowClass::INPUT_ONLY
gtk3: fix a symbol for Gdk::Window::WindowClass::INPUT_ONLY

:      only ->
:input_only
 ^^^^^^

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