Always pack small_vector

Always pack small_vector

Summary:
The check was overly restrictive, the conditional packing was originally used to limit it to GCC only (as it used the `__attribute__` directly), but that restriction is no longer needed as `FOLLY_PACK_*` will be defined appropriately for whatever platform we are currently on.

This just switches it to unconditionally use the `FOLLY_PACK_*` macros rather than defining it's own `FB_PACK_*` macros.

See https://github.com/facebook/folly/commit/aafd8ff9151615a8de09682d41d1c29966e4c8fe for where the `FB_PACK_*` macros could originally have been removed.

Reviewed By: yfeldblum

Differential Revision: D3373779

fbshipit-source-id: 9e41b0836f8ba12de4f37aba58c51f841be08b41