benchmark: ignore build artifacts for napi addons

Add `.gitignore` to ignore the `build` directory in a similar way
to the other addons under `benchmark/napi`.

PR-URL: https://github.com/nodejs/node/pull/35970


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>