Allow for RTRlib w/o SSH support

[Matthias Wählisch]

If RTRlib was compiled without SSH support,

CCLD   bgp_btoa
   libbgp.a(bgp_rpki_commands.o): In function `add_ssh_cache':
   /home/ci/cibuild.67/quagga-source/bgpd/bgp_rpki_commands.c:274: undefined reference to `tr_ssh_init'