src: fix SmartOS compilation

Refs: https://github.com/nodejs/node-v8/issues/8
PR-URL: https://github.com/nodejs/node/pull/14730


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>