[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291517 91177308-0d34-0410-b5e6-96231b3b80d8

src/chrono.cpp

@@ -42,7 +42,7 @@
 #include <winapifamily.h>
 #endif
 #else
-#if !defined(CLOCK_REALTIME)
+#if !defined(CLOCK_REALTIME) || !defined(_LIBCXX_USE_CLOCK_GETTIME)
 #include <sys/time.h>        // for gettimeofday and timeval
 #endif // !defined(CLOCK_REALTIME)
 #endif // defined(_LIBCPP_WIN32API)