Make sure that we don't define INET6 when NO_INET6 is defined.

Without this change, when running netstat with a kernel without
INET6 built in, you will get a complain at the end of "netstat -s"
output.

X-MFC:		NO_INET6 was called "NOINET6" on RELENG_5

usr.bin/netstat/Makefile

@@ -5,7 +5,9 @@ PROG=	netstat
 SRCS=	if.c inet.c inet6.c main.c mbuf.c mcast.c mroute.c ipx.c route.c \
 	unix.c atalk.c netgraph.c mroute6.c ipsec.c
 CFLAGS+=-DIPSEC
+.if !defined(NO_INET6)
 CFLAGS+=-DINET6
+.endif
 WARNS?=	2
 NO_WERROR=