Fix build breakage introduced in r216949

The bug shows up on systems that `#define _POSIX_CLOCK_MONOTONIC 0` to indicate
that users of CLOCK_MONOTONIC must check sysconf at runtime.

See: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/unistd.h.html


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@216997 91177308-0d34-0410-b5e6-96231b3b80d8