Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/compiler/dex/backend.h b/compiler/dex/backend.h
index 6f5ba38..7fa8e99 100644
--- a/compiler/dex/backend.h
+++ b/compiler/dex/backend.h
@@ -30,7 +30,7 @@
     virtual CompiledMethod* GetCompiledMethod() = 0;
 
   protected:
-    Backend(ArenaAllocator* arena) : arena_(arena) {};
+    explicit Backend(ArenaAllocator* arena) : arena_(arena) {};
     ArenaAllocator* const arena_;
 
 };  // Class Backend