Add <string> header
Add <string> header

Summary: std::string used - no header included

Differential Revision: D34005846

fbshipit-source-id: 4a420ca4aa3cde4674677a6aeb28f94caeb5bdce

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