src: define getpid() based on OS

94b9948d added unistd.h to src/env.cc in order to use getpid().
However, this doesn't exist on Windows. This commit conditionally
defines getpid() based on the OS.

Fixes: https://github.com/nodejs/node/issues/4145
PR-URL: https://github.com/nodejs/node/pull/4146


Reviewed-By: Brian White <mscdex@mscdex.net>