tls-stack: Flag libneon as supporting OpenSSL

The default `libneon27` package in Debian is built against OpenSSL,
https://packages.debian.org/sid/libneon27



The GnuTLS backend is enabled in the `libneon27-gnutls` binary package.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>