AudioOutput: pass plugin to constructor

Make it "const".

src/output/plugins/NullOutputPlugin.cxx

@@ -29,9 +29,11 @@ struct NullOutput {
 
 	Timer *timer;
 
+	NullOutput()
+		:base(null_output_plugin) {}
+
 	bool Initialize(const config_param &param, Error &error) {
-		return ao_base_init(&base, &null_output_plugin, param,
-				    error);
+		return ao_base_init(&base, param, error);
 	}
 };