Add time_based option

This allows fio to keep running, even if the workload has completed.
It will simply restart the workload over and over, for as long as the
runtime setting allows.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 9490edc..92264d9 100644
--- a/fio.h
+++ b/fio.h
@@ -413,6 +413,7 @@
 	unsigned int group_reporting;
 	unsigned int fadvise_hint;
 	unsigned int zero_buffers;
+	unsigned int time_based;
 
 	char *read_iolog_file;
 	char *write_iolog_file;