Remove compiling warning.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2348 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/drivers/usb/usbdevice/class/cdc_vcom.c

@@ -14,6 +14,8 @@
 
 #include <rtthread.h>
 #include <rtdevice.h>
+#include <rthw.h>
+ 
 #include "cdc.h"
 
 static uclass_t cdc;