add Lock to struct Ref.

include/thread.h

diff --git a/include/thread.h b/include/thread.h
index 0eb02b5..b4a1c94 100644
--- a/include/thread.h
+++ b/include/thread.h
@@ -60,6 +60,7 @@
 };
 
 struct Ref {
+	Lock lk;
 	long ref;
 };