fix compilation under FreeBSD
fix compilation under FreeBSD

Summary:
struct xucred contains cr_pid on FreeBSD. Unlike Darwin, there's no
need for a separate LOCAL_PEEREPID call.

Reviewed By: yfeldblum

Differential Revision: D33932367

fbshipit-source-id: b64fcaec4382d197baf40e6a781b1fd78a43d8ba

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift