sysutils/unquote: 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:	jrm
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D37504