AudioOutput: pass plugin to constructor

Make it "const".

src/output/plugins/OpenALOutputPlugin.cxx

@@ -48,9 +48,11 @@ struct OpenALOutput {
 	ALenum format;
 	ALuint frequency;
 
+	OpenALOutput()
+		:base(openal_output_plugin) {}
+
 	bool Initialize(const config_param &param, Error &error_r) {
-		return ao_base_init(&base, &openal_output_plugin, param,
-				    error_r);
+		return ao_base_init(&base, param, error_r);
 	}
 };