Revert "Fix blobstore compilation on macOS and MSVC"

This reverts commit ced948046e121392735f42faad8cf304d9ddfd98.
diff --git a/demo/include/blobstore.h b/demo/include/blobstore.h
index 68a7fc2..d89583a 100644
--- a/demo/include/blobstore.h
+++ b/demo/include/blobstore.h
@@ -16,8 +16,8 @@
   BlobMetadata metadata(uint64_t blobid) const;
 
 private:
-  class Impl;
-  std::shared_ptr<Impl> impl;
+  class impl;
+  std::shared_ptr<impl> impl;
 };
 
 std::unique_ptr<BlobstoreClient> new_blobstore_client();