It’s available in jessie-backports since 2016-07-02, see https://packages.debian.org/jessie-backports/libssl-dev

Given that jessie is stable, OpenSSL will not be updated to a newer version, only security updates will be made available.

What I’m trying to say: you’re never going to get it on jessie, unless you enable backports, at which point you’ll have it readily available.

Hope that helps