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/headerctrl.cpp

@@ -268,7 +268,7 @@ void wxHeaderCtrl::DoInsertItem(const wxHeaderColumn& col, unsigned int idx)
     // notice that we need to store the string we use the pointer to until we
     // pass it to the control
     hdi.mask |= HDI_TEXT;
-    wxWxCharBuffer buf = col.GetTitle().wx_str();
+    wxWxCharBuffer buf = col.GetTitle().t_str();
     hdi.pszText = buf.data();
     hdi.cchTextMax = wxStrlen(buf);