apparmor: test: add MODULE_DESCRIPTION()

Fix the 'make W=1' warning:
WARNING: modpost: missing MODULE_DESCRIPTION() in security/apparmor/apparmor_policy_unpack_test.o

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>