checkpolicy/fuzz: Update check_level() to use notdefined field

Commit fe16f58 (Fix potential double free of mls_level_t) renamed
the "defined" field to "notdefined", but did not update this use of
it.

Use the new field "notdefined" for the check.

Signed-off-by: James Carter <jwcart2@gmail.com>

checkpolicy/fuzz/checkpolicy-fuzzer.c

@@ -135,7 +135,7 @@ static int check_level(hashtab_key_t key, hashtab_datum_t datum, void *arg __att
 	const level_datum_t *levdatum = (level_datum_t *) datum;
 
 	// TODO: drop member defined if proven to be always set
-	if (!levdatum->isalias && !levdatum->defined) {
+	if (!levdatum->isalias && levdatum->notdefined) {
 		fprintf(stderr,
 			"Error:  sensitivity %s was not used in a level definition!\n",
 			key);