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/spinctrl.cpp

@@ -452,7 +452,7 @@ void  wxSpinCtrl::SetValue(int val)
         // to leave it like this, while we really want to always show the
         // current value in the control, so do it manually
         ::SetWindowText(GetBuddyHwnd(),
-                        wxString::Format(wxT("%d"), val).wx_str());
+                        wxString::Format(wxT("%d"), val).t_str());
     }
 
     m_oldValue = GetValue();