dtc: Ensure it is built in C++17 mode

Explicitly specify the C++ standard to be used in the Makefile.
This prevents macOS cross-builds from using the default gnu++98 and
fail. This syncs dtc with upstream commit
39a58cfaab7d55c7975ebf905d859ba91a369fa0.

Reviewed by: emaste
Fixes: 29a55fd09b0a ("dtc: Sync with upstream commit 26a0fe5")
Differential Revision: https://reviews.freebsd.org/D42006