Include sys/types.h portability header in Enumerate
Include sys/types.h portability header in Enumerate

Summary: Because it's using `ssize_t`, which MSVC defines weirdly.

Reviewed By: yfeldblum

Differential Revision: D3614050

fbshipit-source-id: 422621ea0dc92b374fa6dfc32a5391c1093d039e

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