src: include signal.h in util.h

It is required for using the "SIGABRT" constant.

It doesn't cause compilation errors in Node because most files already
have "signal.h" included, but it causes errors for third party embedder.

PR-URL: https://github.com/nodejs/node/pull/3058


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Rod Vagg <r@va.gg>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>