Summary:
Avoids -Wundef on non-Windows builds.
folly/system/ThreadId.h had #elif _WIN32 which was changed to
#elif defined(_WIN32)

Reviewed By: Orvid

Differential Revision: D17399760

fbshipit-source-id: 76583374c33d5d59e7c64b9a7a05c660a817bb92