src: make root_cert_vector function scoped

root_cert_vector currently has file scope and external linkage, but is
only used in the NewRootCertsStore function. If this is not required to
be externally linked perhaps it can be changed to be static and function
scoped instead.

PR-URL: https://github.com/nodejs/node/pull/12788


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@ohtsu.org>