gnutls: security bump to version 3.2.20

Fixes:
CVE-2014-8564 / GNUTLS-SA-2014-5 - Sean Burford reported that the
encoding of elliptic curves parameters GnuTLS 3 is vulnerable to a
denial of service (heap corruption). It affects clients and servers
which print information about the peer's certificate, e.g., the key ID,
and can be exploited via a specially crafted X.509 certificate.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>