upnp: fix constructor

Change-Id: I6c8082c7452c371d3c1afc03846452cf0d547e20
diff --git a/include/upnp/upnp_context.h b/include/upnp/upnp_context.h
index bd1cc2b..a486eb5 100644
--- a/include/upnp/upnp_context.h
+++ b/include/upnp/upnp_context.h
@@ -105,7 +105,7 @@
     };
 
 public:
-    UPnPContext(std::shared_ptr<asio::io_context> ctx, std::shared_ptr<dht::log::Logger> logger);
+    UPnPContext(const std::shared_ptr<asio::io_context>& ctx, const std::shared_ptr<dht::log::Logger>& logger);
     ~UPnPContext();
 
     // Retrieve the UPnPContext singleton.