tracing: Fix event header writeback.h to include tracepoint.h

The trace event headers are required to include tracepoint.h. The only reason
they worked now is because module.h included tracepoint.h, and that will soon
change.

Link: http://lkml.kernel.org/r/20140226190644.442886305@goodmis.org

Fixes: 455b286468 "writeback: Initial tracing support"
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

include/trace/events/writeback.h

@@ -4,6 +4,7 @@
 #if !defined(_TRACE_WRITEBACK_H) || defined(TRACE_HEADER_MULTI_READ)
 #define _TRACE_WRITEBACK_H
 
+#include <linux/tracepoint.h>
 #include <linux/backing-dev.h>
 #include <linux/writeback.h>