fix wxClientDisplayRect() when wxUSE_DISPLAY==0
GDK_WINDOWING_X11 was not getting defined, causing hard coded size to be used


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76348 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775