Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/thread_pool.h b/runtime/thread_pool.h
index 3462d5e..b9f185d 100644
--- a/runtime/thread_pool.h
+++ b/runtime/thread_pool.h
@@ -76,7 +76,7 @@
   // after running it, it is the caller's responsibility.
   void AddTask(Thread* self, Task* task);
 
-  ThreadPool(size_t num_threads);
+  explicit ThreadPool(size_t num_threads);
   virtual ~ThreadPool();
 
   // Wait for all tasks currently on queue to get completed.
@@ -161,7 +161,7 @@
 
 class WorkStealingThreadPool : public ThreadPool {
  public:
-  WorkStealingThreadPool(size_t num_threads);
+  explicit WorkStealingThreadPool(size_t num_threads);
   virtual ~WorkStealingThreadPool();
 
  private: