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

wangle/acceptor/TransportInfo.cpp

@@ -46,7 +46,11 @@ bool TransportInfo::initWithSocket(const folly::AsyncSocket* sock) {
   rtt = microseconds(tcpinfo.tcpi_rtt);
   rtt_var = tcpinfo.tcpi_rttvar;
   rto = tcpinfo.tcpi_rto;
+#ifdef __FreeBSD__
+  rtx_tm = tcpinfo.__tcpi_retransmits;
+#else
   rtx_tm = tcpinfo.tcpi_retransmits;
+#endif
   mss = tcpinfo.tcpi_snd_mss;
   cwnd = tcpinfo.tcpi_snd_cwnd;
   cwndBytes = cwnd * mss;