OSX should use off_t for off64_t

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/os/os-mac.h b/os/os-mac.h
index 1c3798b..be25458 100644
--- a/os/os-mac.h
+++ b/os/os-mac.h
@@ -20,6 +20,7 @@
 
 typedef unsigned long os_cpu_mask_t;
 typedef unsigned int clockid_t;
+typedef off_t off64_t;
 
 static inline int blockdev_invalidate_cache(int fd)
 {