net/tls: Fix compilation with older GnuTLS versions
net/tls: Fix compilation with older GnuTLS versions

The GnuTLS developers renamed the GNUTLS_SEC_PARAM_NORMAL enumeration:

commit a20027bc058bbc38d6c7dea76a26dda5d5aa54df
Author: Nikos Mavrogiannopoulos <nmav@redhat.com>
Date:   Tue Jan 14 10:45:21 2014 +0100

    GNUTLS_SEC_PARAM_NORMAL was renamed to GNUTLS_SEC_PARAM_MEDIUM

    That was done to avoid confusion with the NORMAL priority string.
    Also when setting a PROFILE explicitly as priority string the
    session security level is adjusted accordingly.

That causes Seastar build to fail with older GnuTLS version that is
present in Ubuntu 14.04.

Reported-by: Sandeep Joshi <fcfb4160b5cc9fff523463a03cf89b7869564bfb@gmail.com>
Message-Id: <e53a47f088aa4cfe147e7f5bd64fd341f760b57d@scylladb.com>

dreamsxin/seastar,kjniemi/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,kjniemi/seastar,syuu1228/seastar,kjniemi/seastar,scylladb/seastar,cloudius-systems/seastar,scylladb/seastar,cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar