nfc: annotate af_nfc_exit() as __exit

The af_nfc_exit() is used only in other __exit annotated context
(nfc_exit()).
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/nfc/af_nfc.c

@@ -79,7 +79,7 @@ int __init af_nfc_init(void)
 	return sock_register(&nfc_sock_family_ops);
 }
 
-void af_nfc_exit(void)
+void __exit af_nfc_exit(void)
 {
 	sock_unregister(PF_NFC);
 }