Fix build error:

In file included from /home/abuild/rpmbuild/BUILD/llvm/projects/compiler-rt/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc:15:
../projects/compiler-rt/lib/xray/tests/../xray_fdr_logging_impl.h:221:21: error: use of undeclared identifier 'CLOCK_MONOTONIC'
  wall_clock_reader(CLOCK_MONOTONIC, &TS);
                    ^
1 error generated.

llvm-svn: 298837