Contribs: Update GNUTls version to 2.12.23

Hopefully the last 2.12 version?
(cherry picked from commit e68d07da9a54a5885781b91f2a89a28bf40a389e)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/gnutls/SHA512SUMS

-49fde72d049ca75d3df15744519296a5eb982560d739cf9b41033b758564df59698be857bc86825f1ae2dc71b3479208701482bfc085f22e9ff197a65a00d7d1  gnutls-2.12.20.tar.bz2
+7780e9ca7b592350ce9b11e53a63d3212320402d8ad2462bfbc0e69aec4a48bb372a1925627abb7996535c87c90e3d79537ea118c8bb36d26aae8e19eaae3a06  gnutls-2.12.23.tar.bz2

contrib/src/gnutls/rules.mak

 # GnuTLS
 
-GNUTLS_VERSION := 2.12.20
-GNUTLS_URL := http://ftp.gnu.org/pub/gnu/gnutls/gnutls-$(GNUTLS_VERSION).tar.bz2
+GNUTLS_VERSION := 2.12.23
+GNUTLS_URL := ftp://ftp.gnutls.org/gcrypt/gnutls/v2.12/gnutls-$(GNUTLS_VERSION).tar.bz2
 
 PKGS += gnutls
 ifeq ($(call need_pkg,"gnutls >= 2.0.0"),)