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>

content/concepts/tls-stack.md

@@ -514,7 +514,7 @@ appropriate by the individual projects.
 <td>neon27</td>
 <td>LGPL-2.1+</td>
 <td></td>
-<td></td>
+<td>X</td>
 <td>X</td>
 <td></td>
 </tr>