Fix C10_API/C10_EXPORT for op schema registration (#15324)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15324
This was missing but needs to be here, otherwise we can't register schemas without linker errors.
Reviewed By: ezyang
Differential Revision: D13500679
fbshipit-source-id: ba06351cb8ae09ec456cb93e527d388ace578fbb
