[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