Constify a few more hot paths

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/engines/sync.c b/engines/sync.c
index 1329946..41612df 100644
--- a/engines/sync.c
+++ b/engines/sync.c
@@ -138,7 +138,7 @@
 
 static int fio_vsyncio_getevents(struct thread_data *td, unsigned int min,
 				 unsigned int max,
-				 struct timespec fio_unused *t)
+				 const struct timespec fio_unused *t)
 {
 	struct syncio_data *sd = td->io_ops->data;
 	int ret;