Restore access to wxLogStderr::m_fp from derived classes

This was allowed before, but m_fp became private after the refactoring
of a7dddd9f3b.

Make it protected again now to avoid breaking existing code that
accesses it.

include/wx/log.h

@@ -718,7 +718,7 @@ private:
 
 // log everything to a "FILE *", stderr by default
 class WXDLLIMPEXP_BASE wxLogStderr : public wxLog,
-                                     private wxMessageOutputStderr
+                                     protected wxMessageOutputStderr
 {
 public:
     // redirect log output to a FILE