apparmor: add ro rule for sasl GSSAPI plugin on /etc/gss/mech.d/

apparmor: add ro rule for sasl GSSAPI plugin on /etc/gss/mech.d/

If a system has sasl GSSAPI plugin available qemu with sasl support will
try to read /etc/gss/mech.d/.

It is required to allow that to let the modules fully work and it should
be safe to do so as it only registers/configures plugins but has no secrets.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Jamie Strandboge <jamie@canonical.com>
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>