usr.bin/dtc: Include <limits> for std::numeric_limits<>.

This is needed when building natively as a cross-tool on hosts such as
Linux using more recent versions of libstdc++.

Co-authored-by: Alexander Richardson <arichardson@FreeBSD.org>
Obtained from: CheriBSD

(cherry picked from commit 1c44d2bf86834a7214e0cc7bde7c956b5b02fd21)