Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index c73c1bb..541c916 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -456,7 +456,7 @@
   } while (false)
 
  public:
-  WatchDog(bool is_watch_dog_enabled) {
+  explicit WatchDog(bool is_watch_dog_enabled) {
     is_watch_dog_enabled_ = is_watch_dog_enabled;
     if (!is_watch_dog_enabled_) {
       return;