Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/monitor.h b/runtime/monitor.h
index 9206131..3b06217 100644
--- a/runtime/monitor.h
+++ b/runtime/monitor.h
@@ -196,7 +196,7 @@
 // For use only by the JDWP implementation.
 class MonitorInfo {
  public:
-  MonitorInfo(mirror::Object* o) EXCLUSIVE_LOCKS_REQUIRED(Locks::mutator_lock_);
+  explicit MonitorInfo(mirror::Object* o) EXCLUSIVE_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   Thread* owner;
   size_t entry_count;