writeback.h: needs linux/device.h

include/trace/events/writeback.h uses dev_name(), so it needs to
include linux/device.h.

include/trace/events/writeback.h:12: error: implicit declaration of function 'dev_name'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

include/trace/events/writeback.h

@@ -5,6 +5,7 @@
 #define _TRACE_WRITEBACK_H
 
 #include <linux/backing-dev.h>
+#include <linux/device.h>
 #include <linux/writeback.h>
 
 struct wb_writeback_work;