openssh: Add ${SKSRCS} to libssh

[jlduran]

Summary

FreeBSD prefixes libssh symbols with "Fssh_" in ssh_namespace.h. This
header file is included in all targets that consume libssh (in
secure/ssh.mk). It needs access to all the definition implementations,
some of them are in ${SKSRCS}.

Note that this is a FreeBSD-specific requirement, that is why we put it
in a separate sources line.

Fixes: 65d8491719bb ("secure: Adapt Makefile to ssh-sk-client everywhere")

[jlduran]

This should fix sh freebsd-namespace.sh and technically, we can (should?) now revert a098111a28ed59e1ab1101ad09913f0235ebd28f.
In my opinion this is the least obstructive way forward.

Thank you @jrtc27 and @lattera-gmail.com for the observations, and @jhb for checking against gcc.