Fix build on Unix systems without POSIX realtime extensions.

This allows BoringSSL to build on OpenBSD with gcc/g++ 4.9.2.

Change-Id: Icce23de87b0358a581124eb8cd37dc48a1f096c9
Reviewed-on: https://boringssl-review.googlesource.com/5401
Reviewed-by: Adam Langley <agl@google.com>
1 file changed