thrift: macOS doesn't have struct iphdr, disable PcapLogging there
thrift: macOS doesn't have struct iphdr, disable PcapLogging there

Summary:
Compilation fails on macOS, and a quick read around indicates that it
may be non-trivial to actually support this functionality on bsdish systems,
and it isn't necessary for correct operation so ifdef away the PcapLogger on
that platform.

Reviewed By: yfeldblum

Differential Revision: D3552389

fbshipit-source-id: a84c247809aa5d15e006246ea0654426663de863

LinusU/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,facebook/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,facebook/fbthrift,facebook/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,LinusU/fbthrift,facebook/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,facebook/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift