Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/gc/accounting/heap_bitmap.h b/runtime/gc/accounting/heap_bitmap.h
index a12809e..f4b725c 100644
--- a/runtime/gc/accounting/heap_bitmap.h
+++ b/runtime/gc/accounting/heap_bitmap.h
@@ -103,8 +103,7 @@
   void ReplaceObjectSet(SpaceSetMap* old_set, SpaceSetMap* new_set)
       EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_);
 
-  HeapBitmap(Heap* heap) : heap_(heap) {
-  }
+  explicit HeapBitmap(Heap* heap) : heap_(heap) {}
 
  private: