mail/milter-skem: disable date/time nonreproducible build warning

The base system build infrastructure will add -Wdate-time, to avoid
use of nonreproducible DATE and TIME in the base system.

This port makes use of the base system build infrastructure as well as
DATE and/or TIME, so explicitly opt out of the warning.

PR: 267902 [exp-run]
Approved by: mi
Sponsored By: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37507