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>

security/apparmor/lsm.c

@@ -2106,7 +2106,7 @@ __initcall(apparmor_nf_ip_init);
 static char nulldfa_src[] = {
 	#include "nulldfa.in"
 };
-struct aa_dfa *nulldfa;
+static struct aa_dfa *nulldfa;
 
 static char stacksplitdfa_src[] = {
 	#include "stacksplitdfa.in"