[Chrono][Darwin] Include header for gettimeofday

Followup on r291466 and include the proper header. This fixes:
https://build.chromium.org/p/chromium.fyi/builders/ClangToTMac/builds/12620/steps/gclient%20runhooks/logs/stdio

llvm-svn: 291517
1 file changed