tcp md5: improve consistency

All other usages of SCF_SIGNATURE are protected by IPSEC_SUPPORT
or TCP_SIGNATURE.
No functional change intended.

Reported by:	Hannes Elfert
MFC after:	1 week
MFC to:		stable/14
MFC to:		stable/15

sys/netinet/tcp_syncache.c

@@ -2385,8 +2385,10 @@ syncookie_expand(struct in_conninfo *inc, const struct syncache_head *sch,
 		sc->sc_tsoff = tcp_new_ts_offset(inc);
 	}
 
+#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 	if (to->to_flags & TOF_SIGNATURE)
 		sc->sc_flags |= SCF_SIGNATURE;
+#endif
 
 	sc->sc_rxmits = 0;