Build static extension_module lib for iOS demo apps (#2038)

Summary:
Building the demo apps on iOS is broken after https://github.com/pytorch/executorch/pull/2022 because it switches the `extension_module` library from a static to a shared one.  AFAIK, the latter requires code signing and it's failing on CI with the following error `Signing for "extension_module" requires a development team`

I'm not quite sure what would be the best way to address this, but keeping the library as a static one for iOS build seems like an easy workaround.

Pull Request resolved: https://github.com/pytorch/executorch/pull/2038

Reviewed By: larryliu0820

Differential Revision: D54047521

Pulled By: huydhn

fbshipit-source-id: e752f29bb1fefd355525a40d586f89659119a625