Bumping gnutls dependency to 2.12.8

[Stef Walter <stefw collabora co uk>]

Hey all,

At the beginning of the 3.3 cycle, I bumped the gnutls minimum
dependency to 2.12.0. It turns out that release had some issues, it
doesn't even have the right version in the pkg-config file.

So unless anyone objects, I'm going to bump the jhbuild 3.4 moduleset
gnutls version to 2.12.8.

As a bonus, this is the minimum version we need for the PKCS#11
integration in glib-networking to function.

Cheers,

Stef