ntp: security update, and use libcrypto

http://support.ntp.org/bin/view/Main/SecurityNotice#Recent_Vulnerabilities

The package would no longer build without libcrypto,
and it wouldn't find it without pkgconfig.
I checked that Debian and Arch do use openssl as a dependency,
so it's probably not so bad a thing to have.

CC maintainer @edolstra.