Fix build on XCode 8 w/OSX 10.12 SDK

CLOCK_REALTIME (along with clock_gettime and friends) are now defined,
so the build fails with a redefinition error.

Test: m checkbuild (w/XCode 8 on 10.11 with 10.12 SDK)
Change-Id: Ic8a5cc872ded1766597fda5f098d0e044cd063ee
1 file changed