Add functions to read MTP events from MTP devices.

BUG=23368533

Change-Id: I2003dda961339677caf56e57ae90cf6df7bd7430
diff --git a/media/mtp/MtpPacket.h b/media/mtp/MtpPacket.h
index 037722a..0e96309 100644
--- a/media/mtp/MtpPacket.h
+++ b/media/mtp/MtpPacket.h
@@ -19,6 +19,7 @@
 
 #include "MtpTypes.h"
 
+struct usb_device;
 struct usb_request;
 
 namespace android {