Update flatcc to 896db54 (#3967)

Summary:
See changes in
https://github.com/dvidelabs/flatcc/compare/eb5228f...896db54

We don't need anything specific from this newer version, but it's good to stay up-to-date to ensure we have the latest bug/security fixes.

Pull Request resolved: https://github.com/pytorch/executorch/pull/3967

Test Plan:
Followed the instructions at
https://pytorch.org/executorch/main/tutorials/sdk-integration-tutorial.html
up to the `print_tabular_data` step of `Creating an Inspector`.

Reviewed By: tarun292

Differential Revision: D58494083

Pulled By: dbort

fbshipit-source-id: 5d6eda6efb1e5a7c9ba1a91845138bf05f258306