package/memcached: add openssl optional dependency

openssl is an optional dependency since version 1.5.13 and
https://github.com/memcached/memcached/commit/ee1cfe3bf9384d1a93545fc942e25bed6437d910


which must be handled through pkg-config to avoid static build failure

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>