gitignore: ignore all tap files

We now have multiple tap producers; just ignore all
files with the `.tap` extension.

PR-URL: https://github.com/nodejs/node/pull/9262


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Myles Borins <myles.borins@gmail.com>