Fix build under 13-ALPHA and 14-CURRENT.

As of src/7fa2f2a62f04f095e1e27ad55aa22a8f59b1df8f NO_WERROR was
renamed MK_WERROR. Rather than add logic, use a redundant definition that
will be a NOP on the "other" version of FreeBSD. This should be maintained
until 12-STABLE is retired.

PR: 253000
Submitted by: cy
Reported by: cy
Reviewed by: jlh (maintainer)
Approved by: jlh (maintainer)
MFH: 2021Q1
Differential Revision: https://reviews.freebsd.org/D28361