Fix ScopedArenaAllocator::Reset() for Create()d allocators.

Change-Id: I88cbb329911ed489768772218b49b6f1756ffd86
diff --git a/compiler/utils/scoped_arena_allocator.h b/compiler/utils/scoped_arena_allocator.h
index 62ea330..523f158 100644
--- a/compiler/utils/scoped_arena_allocator.h
+++ b/compiler/utils/scoped_arena_allocator.h
@@ -132,6 +132,8 @@
   uint8_t* mark_ptr_;
   uint8_t* mark_end_;
 
+  void DoReset();
+
   template <typename T>
   friend class ScopedArenaAllocatorAdapter;