Add missing comparison operator declarations in wxString::iterator.

Add missing comparison operator declarations in wxString::iterator.

Fix compilation in !wxUSE_UNICODE_UTF8 case after r65857.

Modify the second declaration of wxString::iterator class which was not
updated by the previous commit in the same way, i.e. add declaration of
comparison operators taking const_iterator to iterator class.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65859 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775