time: convert to uint64_t

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/time.c b/time.c
index c4d1d4c..17f9f6f 100644
--- a/time.c
+++ b/time.c
@@ -53,7 +53,7 @@
 	} while (!td->terminate);
 }
 
-unsigned long mtime_since_genesis(void)
+uint64_t mtime_since_genesis(void)
 {
 	return mtime_since_now(&genesis);
 }