AudioOutput: pass plugin to constructor

Make it "const".

src/output/plugins/OssOutputPlugin.cxx

@@ -78,11 +78,12 @@ struct OssOutput {
 	 */
 	int oss_format;
 
-	OssOutput():fd(-1), device(nullptr) {}
+	OssOutput()
+		:base(oss_output_plugin),
+		 fd(-1), device(nullptr) {}
 
 	bool Initialize(const config_param &param, Error &error_r) {
-		return ao_base_init(&base, &oss_output_plugin, param,
-				    error_r);
+		return ao_base_init(&base, param, error_r);
 	}
 };