Compilation fix for the example in wxThread documentation.

Make MyThread a friend to allow it accessing the private fields of MyFrame.

Notice that this example still remains badly written and should be ideally
entirely redone using conditions and mutexes.

See #15881.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75637 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/thread.h

@@ -779,6 +779,8 @@ enum wxThreadError
         MyThread *m_pThread;
         wxCriticalSection m_pThreadCS;    // protects the m_pThread pointer
 
+        friend class MyThread;            // allow it to access our m_pThread
+
         wxDECLARE_EVENT_TABLE();
     };