made it build without dht support

src/session_impl.cpp

@@ -991,6 +991,8 @@ namespace detail
 		}
 	}
 
+#ifndef TORRENT_DISABLE_DHT
+
 	void session_impl::on_receive_udp(udp::endpoint const& ep, char const* buf, int len)
 	{
 		if (len > 20 && *buf == 'd' && m_dht)
@@ -1000,6 +1002,8 @@ namespace detail
 		}
 	}
 
+#endif
+	
 	void session_impl::async_accept(boost::shared_ptr<socket_acceptor> const& listener)
 	{
 		shared_ptr<socket_type> c(new socket_type);