Use wxString::t_str() in calls to Windows API functions in wxMSW.

Use t_str() instead of wx_str() to make the code work correctly in UTF-8 build
in which wx_str() returns a pointer to UTF-8 buffer while we need a wchar_t
pointer for Windows.

Closes #14371.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71640 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/dc.cpp

@@ -1871,7 +1871,7 @@ void wxMSWDCImpl::DoGetTextExtent(const wxString& string, wxCoord *x, wxCoord *y
 
     SIZE sizeRect;
     const size_t len = string.length();
-    if ( !::GetTextExtentPoint32(GetHdc(), string.wx_str(), len, &sizeRect) )
+    if ( !::GetTextExtentPoint32(GetHdc(), string.t_str(), len, &sizeRect) )
     {
         wxLogLastError(wxT("GetTextExtentPoint32()"));
     }