t/lfsr-test: fixup time

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fio_time.h b/fio_time.h
index c550a55..9f7d209 100644
--- a/fio_time.h
+++ b/fio_time.h
@@ -1,6 +1,7 @@
 #ifndef FIO_TIME_H
 #define FIO_TIME_H
 
+struct thread_data;
 extern uint64_t utime_since(struct timeval *, struct timeval *);
 extern uint64_t utime_since_now(struct timeval *);
 extern uint64_t mtime_since(struct timeval *, struct timeval *);