Specialize std::hash<> for wxString when using C++11.

This allows to use wxString as key type of std::unordered_{map,hash} out of
the box.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77518 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775