Summary:
Fixes compile error:

In file included from ./util/statistics.h:17:0,
                 from ./util/stop_watch.h:8,
                 from ./util/perf_step_timer.h:9,
                 from ./util/iostats_context_imp.h:8,
                 from ./util/posix_logger.h:27,
                 from ./port/util_logger.h:18,
                 from ./db/auto_roll_logger.h:15,
                 from db/auto_roll_logger.cc:6:
./util/thread_local.h:65:16: error: 'function' in namespace 'std' does not name a template type
   typedef std::function<void(void*, void*)> FoldFunc;
Closes https://github.com/facebook/rocksdb/pull/1656

Differential Revision: D4318702

Pulled By: yiwu-arbug

fbshipit-source-id: 8c5d17a