divert: unbreak the LINT-NOIP build

Only expose `dcb` when either `INET` or `INET6` is defined.

Reported by:	clang (`-Wunused`)
MFC after:	1 week
Fixes 5547a7bb39 ("divert: Use a better source identifier...")
Differential Revision: https://reviews.freebsd.org/D55548

(cherry picked from commit bd758ebac3272f720c051570e043e4bc653dcd48)

sys/netinet/ip_divert.c

@@ -530,10 +530,11 @@ static int
 div_output_inbound(int family, struct socket *so, struct mbuf *m,
     struct sockaddr_in *sin)
 {
-	struct divcb *dcb;
+#if defined(INET) || defined(INET6)
+	struct divcb *dcb = so->so_pcb;
+#endif
 	struct ifaddr *ifa;
 
-	dcb = so->so_pcb;
 	if (m->m_pkthdr.rcvif == NULL) {
 		/*
 		 * No luck with the name, check by IP address.