add sound device definition.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@334 bbd45198-f89e-11dd-88c7-29a3b14d5316

include/rtdef.h

@@ -532,6 +532,7 @@ enum rt_device_class_type
 	RT_Device_Class_MTD,		/* memory device */
 	RT_Device_Class_CAN,		/* CAN device */
 	RT_Device_Class_RTC,		/* RTC device */
+	RT_Device_Class_Sound,		/* Sound device */
 	RT_Device_Class_Unknown		/* unknown device */
 };