Add missing dep to executorch/extensin/pybindings:portable_lib (#8885)

This file clearly imports executorch.exir._warnings and thus it should have the dep.

Differential Revision: [D70451304](https://our.internmc.facebook.com/intern/diff/D70451304/)

ghstack-source-id: 269199558
Pull Request resolved: https://github.com/pytorch/executorch/pull/8871

Co-authored-by: Scott Wolchok <swolchok@meta.com>