Makefile: fix targets run-server & run-client with GnuTLS 3.5.9
* Makefile (keydir/tls-privkey.pem keydir/tls-pubkey.pem): If the TLS session keys were not generated by mandos-keygen (which will happen with GnuTLS version 3.5.9 or older), create dummy empty unused files to avoid confusing make(1), which would otherwise re-run this target and create new OpenPGP key files which would not match the key fingerprint in the confdir/clients.conf file.