tcpdump: Initialize tzcode early.

An explicit tzset() call is usually not needed as it happens implicitly
the first time we call localtime() or mktime(), but in some cases
(sandboxing, chroot) this may be too late.

PR: 273807
MFC after: 3 days
Reviewed by: jrm
Differential Revision: https://reviews.freebsd.org/D41880