src: define AI_V4MAPPED for OpenBSD
src: define AI_V4MAPPED for OpenBSD

OpenBSD doesn't define AI_V4MAPPED by default. Defining it to 0
as done with other ports.

PR-URL: https://github.com/iojs/io.js/pull/427
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node