clockperf.c: check if HAVE_CLOCK_GETTIME is defined before using CLOCK_MONOTONIC
clockperf.c: check if HAVE_CLOCK_GETTIME is defined before using CLOCK_MONOTONIC

Signed-off-by: Steven Noonan <4068f0880b399410602d694b3cc711c8a8f4727e@uplinklabs.net>

tycho/clockperf,tycho/clockperf