apparmor: add label data availability to the feature set

gsettings mediation needs to be able to determine if apparmor supports
label data queries. A label data query can be done to test for support
but its failure is indistinguishable from other failures, making it an
unreliable indicator.

Fix by making support of label data queries available as a flag in the
apparmorfs features dir tree.

Signed-off-by: John Johansen <john.johansen@canonical.com>