fix compilation under FreeBSD
fix compilation under FreeBSD

Summary:
tcpi_retransmits has two underscores in front on FreeBSD. Maybe it's
deprecated?

Reviewed By: yfeldblum

Differential Revision: D33932259

fbshipit-source-id: 3ddf5fb39b43600e66796759cc59d72628a52694

facebook/wangle,facebook/wangle,facebook/wangle