revert change

Author: paleolimbot

src/nanoarrow/testing/testing.cc

@@ -34,8 +34,7 @@ namespace internal {
 // Internal representation of the various structures needed to import and/or export
 // a dictionary array. We use a serialized version of the dictionary value because
 // nanoarrow doesn't currently have the ability to copy or reference count an Array.
-class Dictionary {
- public:
+struct Dictionary {
   nanoarrow::UniqueSchema schema;
   int64_t column_length;
   std::string column_json;