THRIFT-2091 Unnecessary 'friend' declaration causes warning in TWinsockSingleton

Patch: Konrad Grochowski
diff --git a/lib/cpp/src/thrift/windows/TWinsockSingleton.h b/lib/cpp/src/thrift/windows/TWinsockSingleton.h
index 5f9b6e2..ab12c22 100644
--- a/lib/cpp/src/thrift/windows/TWinsockSingleton.h
+++ b/lib/cpp/src/thrift/windows/TWinsockSingleton.h
@@ -57,12 +57,6 @@
 
 private:
 
-#if USE_BOOST_THREAD
-    friend void boost::call_once(void (*func)(void), boost::once_flag& flag);
-#endif
-
-private:
-
     TWinsockSingleton(void);
 
 public: