Compilation fix for wxUSE_STD_STRING==0 build.

Add missing check for wxUSE_STD_STRING to fix the build with it set to 0 after
the changes of r77518.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77529 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775