benchmark: fix getStringWidth() benchmark

8fb5fe28 broke the benchmark for
getStringWidth(). This fixes it up by updating the argument to
`require()` to retrieve `getStringWidth()` from the new internal module
location.

PR-URL: https://github.com/nodejs/node/pull/31476


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>