output: move functions into the AudioOutput struct

src/output/plugins/OssOutputPlugin.cxx

@@ -83,7 +83,7 @@ struct OssOutput {
 		 fd(-1), device(nullptr) {}
 
 	bool Initialize(const config_param &param, Error &error_r) {
-		return ao_base_init(&base, param, error_r);
+		return base.Configure(param, error_r);
 	}
 };