Smack: Fix kbuild reported build error

The variable sap is defined under ifdef, but a recently
added use of the variable was not. Put that use under ifdef
as well.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack_lsm.c

@@ -3693,6 +3693,7 @@ static int smack_socket_sendmsg(struct socket *sock, struct msghdr *msg,
 			return -EINVAL;
 		rc = smack_netlabel_send(sock->sk, sip);
 		break;
+#if IS_ENABLED(CONFIG_IPV6)
 	case AF_INET6:
 		if (msg->msg_namelen < SIN6_LEN_RFC2133 ||
 		    sap->sin6_family != AF_INET6)
@@ -3706,6 +3707,7 @@ static int smack_socket_sendmsg(struct socket *sock, struct msghdr *msg,
 #ifdef SMACK_IPV6_PORT_LABELING
 		rc = smk_ipv6_port_check(sock->sk, sap, SMK_SENDING);
 #endif
+#endif /* IS_ENABLED(CONFIG_IPV6) */
 		break;
 	}
 	return rc;