Include the atomic portability header in AtomicHashMapTest
Include the atomic portability header in AtomicHashMapTest

Summary: It's using `__sync_fetch_and_add`, so include the right portability header.

Reviewed By: yfeldblum

Differential Revision: D13037888

fbshipit-source-id: 72b209abcf4180cd480be99be20893f74f181e83

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly