Avoid useless work: Do not build inet6.c if INET6 support is off.

This also avoids pretending that netstat includes inet6.c in the
output from ident(1).

usr.bin/netstat/Makefile

@@ -4,7 +4,7 @@
 .include <bsd.own.mk>
 
 PROG=	netstat
-SRCS=	if.c inet.c inet6.c main.c mbuf.c mcast.c mroute.c route.c \
+SRCS=	if.c inet.c main.c mbuf.c mcast.c mroute.c route.c \
 	unix.c atalk.c netgraph.c mroute6.c ipsec.c bpf.c pfkey.c
 
 WARNS?=	2
@@ -12,7 +12,9 @@ NO_WERROR=
 
 CFLAGS+=-DIPSEC
 CFLAGS+=-DFAST_IPSEC
+
 .if ${MK_INET6_SUPPORT} != "no"
+SRCS+=	inet6.c
 CFLAGS+=-DINET6
 .endif