Don't export util/mktar.sh

When creating a tarball, it's pointless to include scripts that assume
a git workspace.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7696)

(cherry picked from commit b9a694717902af796639e1dff641ba620703303b)