Fix extension/data_loader installation (#3355)

Summary:
`libextension_data_loader.a` is not installed properly. This PR removes the prefix so that it can be properly installed

Pull Request resolved: https://github.com/pytorch/executorch/pull/3355

Test Plan: See `libextension_data_loader.a` showing up under executorch/cmake-out/lib.

Reviewed By: lucylq, mikekgfb

Differential Revision: D56580943

Pulled By: larryliu0820

fbshipit-source-id: b771192d03799fd576e8591ec7c45fae23f20762