Add missing 'explicit's on single-argument constructors.

Change-Id: I1494df6e74ec16238971fb4346ba184eb61c37ab
diff --git a/src/monitor.h b/src/monitor.h
index 1316532..459cb04 100644
--- a/src/monitor.h
+++ b/src/monitor.h
@@ -79,7 +79,7 @@
   Object* GetObject();
 
  private:
-  Monitor(Object* obj);
+  explicit Monitor(Object* obj);
 
   void AppendToWaitSet(Thread* thread);
   void RemoveFromWaitSet(Thread* thread);