BUILD: ssl: fix build error introduced by recent commit

BUILD: ssl: fix build error introduced by recent commit

Commit d2cab92 ("BUG/MINOR: ssl: fix management of the cache where forged
certificates are stored") removed some needed #ifdefs resulting in ssl not
building on older openssl versions where SSL_CTRL_SET_TLSEXT_HOSTNAME is
not defined :

src/ssl_sock.c: In function 'ssl_sock_load_ca':
src/ssl_sock.c:2504: error: 'ssl_ctx_lru_tree' undeclared (first use in this function)
src/ssl_sock.c:2504: error: (Each undeclared identifier is reported only once
src/ssl_sock.c:2504: error: for each function it appears in.)
src/ssl_sock.c:2505: error: 'ssl_ctx_lru_seed' undeclared (first use in this function)
src/ssl_sock.c: In function 'ssl_sock_close':
src/ssl_sock.c:3095: error: 'ssl_ctx_lru_tree' undeclared (first use in this function)
src/ssl_sock.c: In function '__ssl_sock_deinit':
src/ssl_sock.c:5367: error: 'ssl_ctx_lru_tree' undeclared (first use in this function)
make: *** [src/ssl_sock.o] Error 1

Reintroduce the ifdefs around the faulty areas.