libssh: correct libssh src file list

Link against the ssh-sk-helper client rather than the sk internal
implementation.

PR: 258384
Tested by: madpilot
Fixes: f448c3ed4ae1 ("openssh: Add new source files to libssl")
Fixes: 19261079b743 ("openssh: update to OpenSSH v8.7p1")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32529