Fix directory iteration in mstch_generator
Fix directory iteration in mstch_generator

Summary:
Old versions of boost don't support using directory iterators in
foreach loops. This fixes the problem which can break the build on certain
platforms.

Reviewed By: yfeldblum

Differential Revision: D3761078

fbshipit-source-id: 8914f5b3bdf982fbb1c457b8b23a2a3193e20b53

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