build: fix `process.platform`

e1fe270f introduces the NODE_PLATFORM macro which had to be redefined in
node.gyp for `process.platform` to return expected values.

PR-URL: https://github.com/iojs/io.js/pull/271


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Bert Belder <bertbelder@gmail.com>