Don't install PDB for Windows static build of caffe2_observers (#16420)
Summary:
Fixes #16292.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16420
Differential Revision: D13833704
Pulled By: soumith
fbshipit-source-id: 482ad6ce103bed7206e924e8c82454fbb1bfac42
