Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/barrier.h b/runtime/barrier.h
index e0ad239..22f08e1 100644
--- a/runtime/barrier.h
+++ b/runtime/barrier.h
@@ -25,7 +25,7 @@
 
 class Barrier {
  public:
-  Barrier(int count);
+  explicit Barrier(int count);
   virtual ~Barrier();
 
   // Pass through the barrier, decrements the count but does not block.