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/toolbar.cpp

@@ -985,7 +985,7 @@ bool wxToolBar::Realize()
                 {
                     const wxString& label = tool->GetLabel();
                     if ( !label.empty() )
-                        button.iString = (INT_PTR)label.wx_str();
+                        button.iString = (INT_PTR) wxMSW_CONV_LPCTSTR(label);
                 }
 
                 button.idCommand = tool->GetId();