apparmor: declare nulldfa as static

With the conversion to a refcounted pdb the nulldfa is now only used
in security/apparmor/lsm.c so declar it as static.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311092038.lqfYnvmf-lkp@intel.com/


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