clock: ensure that we re-init if the clocksource changes from the default

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/options.c b/options.c
index 738c78b..ae290ed 100644
--- a/options.c
+++ b/options.c
@@ -371,6 +371,7 @@
 
 	fio_clock_source = td->o.clocksource;
 	fio_clock_source_set = 1;
+	fio_clock_init();
 	return 0;
 }