libarchive: Staticize some variables

This code was not being built due to errors in our libarchive
configuration. Now that those have been addressed, staticize some
variables that trip a “no previous extern declaration” error. This
is a subset of upstream PR 2962.

MFC after: 1 week
Reviewed by: mm
Differential Revision: https://reviews.freebsd.org/D56471

(cherry picked from commit 7e9d974bc023755161742f66c8c77546bab88586)