[traceroute6] Don't do the casper bits when we're not doing casper

This with the previous patch I committed makes traceroute6/traceroute
compile fine when libcasper isn't enabled.

This complains strongly with unused variables and such when compiled
with gcc-6 on mips32.

Tested:

* compiled/run on mips32 hardware (AR9344)

Reviewed by:	emaste
Differential Revision: https://reviews.freebsd.org/D26773

usr.sbin/traceroute6/traceroute6.c

@@ -1527,8 +1527,9 @@ get_uphdr(struct ip6_hdr *ip6, u_char *lim)
 }
 
 void
-capdns_open()
+capdns_open(void)
 {
+#ifdef	WITH_CASPER
 	const char *types[] = { "NAME", "ADDR" };
 	int families[1];
 	cap_channel_t *casper;
@@ -1545,6 +1546,7 @@ capdns_open()
 	if (cap_dns_family_limit(capdns, families, nitems(families)) < 0)
 		errx(1, "unable to limit access to system.dns service");
 	cap_close(casper);
+#endif	/* WITH_CASPER */
 }
 
 void