apparmor: Fix warning about unused function apparmor_ipv6_postroute

when compiled without CONFIG_IPV6:
security/apparmor/lsm.c:1601:21: warning: ‘apparmor_ipv6_postroute’ defined but not used [-Wunused-function]
 static unsigned int apparmor_ipv6_postroute(void *priv,
                     ^~~~~~~~~~~~~~~~~~~~~~~
Reported-by: NJordan Glover <Golden_Miller83@protonmail.ch>
Tested-by: NJordan Glover <Golden_Miller83@protonmail.ch>
Signed-off-by: NPetr Vorel <pvorel@suse.cz>
Signed-off-by: NJohn Johansen <john.johansen@canonical.com>

security/apparmor/lsm.c

@@ -1599,12 +1599,14 @@ static unsigned int apparmor_ipv4_postroute(void *priv,
 	return apparmor_ip_postroute(priv, skb, state);
 }
 
+#if IS_ENABLED(CONFIG_IPV6)
 static unsigned int apparmor_ipv6_postroute(void *priv,
 					    struct sk_buff *skb,
 					    const struct nf_hook_state *state)
 {
 	return apparmor_ip_postroute(priv, skb, state);
 }
+#endif
 
 static const struct nf_hook_ops apparmor_nf_ops[] = {
 	{