Auto merge of #697 - wezm:openbsd-so_timestamp, r=alexcrichton

Add missing SO_TIMESTAMP constant for OpenBSD like OSes

Added to mod.rs as both [OpenBSD](https://github.com/openbsd/src/blob/e35050970e3e2b822e383a5e844e4c4402eebccc/sys/sys/socket.h#L97) and [Bitrig](https://github.com/bitrig/bitrig/blob/a15267d8249fe8d329eb34048605c59a394a02ac/sys/sys/socket.h#L100) have it.

Required for https://github.com/nix-rust/nix/pull/688