bgpd: fix rpki compilation without rtrlib ssh

bgpd: fix rpki compilation without rtrlib ssh

Fix RPKI module compilation when rtrlib is compiled without SSH support,
ie. with cmake option:

> -D RTRLIB_TRANSPORT_SSH=No

> bgpd/bgp_rpki.c: In function ‘config_write’:
> bgpd/bgp_rpki.c:1062:3: error: enumeration value ‘SSH’ not handled in switch [-Werror=switch-enum]
>  1062 |   switch (cache->type) {
>       |   ^~~~~~
> bgpd/bgp_rpki.c: In function ‘show_rpki_cache_connection_magic’:
> bgpd/bgp_rpki.c:1598:3: error: enumeration value ‘SSH’ not handled in switch [-Werror=switch-enum]
>  1598 |   switch (cache->type) {
>       |   ^~~~~~
> cc1: all warnings being treated as errors

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>