The code using GDK_META_MASK was moved to wx/gtk/private/event.h from
gtk/window.cpp but the fallback definition of GDK_META_MASK for GTK+ < 2.10
remained in window.cpp.
Move it to the header as well now so that it can be compiled with GTK+ < 2.10
again.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69698 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775