src: fix warning in string_search.h

Fixes: https://github.com/nodejs/node/issues/37145

PR-URL: https://github.com/nodejs/node/pull/37146


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>