gnutls: bump to version 3.3.12

This is the new stable, so switch to it.
Still no dice with nettle 3.0, see:
http://lists.gnutls.org/pipermail/gnutls-devel/2014-June/006977.html
and nettle upstream:
http://lists.lysator.liu.se/pipermail/nettle-bugs/2014/003129.html



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