add missing Puppet module puppetlabs/mount_core

We use the 'mount' resource from the 'puppetlabs/mount_core' module (in
tails::profile::mounts), but that module was not present as a Git
submodule before the switch to g10k, so it wasn't added to the new
Puppetfile.

The 'mount' resource source file was cached, though, in the Puppet cache
directory (at /var/cache/lib/puppet/provider/mount.rb) in all relevant
nodes, but it was removed in the first run after the switch to g10k.
That removal from the cache was probably caused because of the
intentional removal of another resource (tails_read_file) which must've
triggered a refresh of the cache.

Let's add the latest version of that module.

refs tpo/tpa/team#42095