MINOR: ssl: make the ssl_fc_sni() sample-fetch function always available

MINOR: ssl: make the ssl_fc_sni() sample-fetch function always available

Its definition is enclosed inside an ifdef SSL_CTRL_SET_TLSEXT_HOSTNAME
which is defined since OpenSSL 0.9.8. Having it conditioned like this
prevents us from using it by default in a log format, which could cause
an error on an old or exotic library.

Let's just always define it and make the sample fetch fail to return
anything on such libs instead.