src: add fcntl.h include to node.cc

https://github.com/nodejs/node/pull/11863 adds _O_RDWR to node.cc
which is defined in fcntl.h. This adds this include directly to
node.cc.

PR-URL: https://github.com/nodejs/node/pull/12540


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Myles Borins <myles.borins@gmail.com>