libevent: add optional dependency on OpenSSL

libevent can optionally use OpenSSL, so add an optional dependency on
this package to explicit this possibility. This makes sure libevent
always gets built with OpenSSL support when the OpenSSL package is
enabled.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>