apparmor: fix regression in network mediation when using feature pinning

When the 4.14-rc6 and earlier kernels are used with an upstream 4.13
or earlier pinned feature set, there is a regression in network
mediation where policy is not being correctly enforced, because the
compilation is completely dropping the af mediation table as expected
by pre 4.14 kernels but the 4.14 kernel is not accounting for this.

Resulting in network denials that can not be fixed by policy.

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