Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/compiler/dex/arena_bit_vector.h b/compiler/dex/arena_bit_vector.h
index 0b7bbc5..7e5c436 100644
--- a/compiler/dex/arena_bit_vector.h
+++ b/compiler/dex/arena_bit_vector.h
@@ -33,7 +33,7 @@
 
     class Iterator {
       public:
-        Iterator(ArenaBitVector* bit_vector)
+        explicit Iterator(ArenaBitVector* bit_vector)
           : p_bits_(bit_vector),
             bit_storage_(bit_vector->GetRawStorage()),
             bit_index_(0),