libarchive: Restore explicit linking to openssl crypto library

This is present in the upstream libarchive CMake code.  We removed it
in commit 59d28b2849 (libarchive: Drop early use of crypto library,
2016-05-03, v3.6.0-rc1~100^2), but didn't record details for why.
Many other changes have been made since then, so the original problem
may no longer occur.  Restore the link dependency since it reflects
a real dependency of the implementation.