p11-kit 0.16.0 recommended dependency

[Stef Walter <stefw gnome org>]

In gnome-keyring 3.7.91 and later versions no longer include a store for
loading root certificate authority anchors from a file. That
functionality was moved into p11-kit 0.16.0 so it can be more widely used.

http://p11-glue.freedesktop.org/trust-module.html

The upshot is that although it's not a hard dependency, gnome-keyring
3.7.91 should be used with p11-kit 0.16.0 or later in order not to have
a feature regression.

I've updated jhbuild to reflect this.

Cheers,

Stef