Compilation fix for wxUSE_UNICODE_WCHAR && !wxUSE_STL_BASED_WXSTRING.

Construct wxScopedWCharBuffer using CreateNonOwned() in
wxString::ToStdWstring() in this case, creating it directly from wc_str()
doesn't work (intentionally, as this doesn't tell if the buffer owns the
string or not).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65928 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775