Don't export private wxDisplayImplX11 class.

This class is only used internally and doesn't need to be exported. In
addition to being unnecessary, this also resulted in ABI compatibility check
errors after changes to this class in 3.0 branch, so avoid such problems in
the future.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77944 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775