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.

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>