Correct mistake in SPDX tag, to BSD-2-Clause
See: https://lists.ntpsec.org/pipermail/devel/2020-April/009293.html
I have reviewed the files with BSD-4-Clause-UC , and verified that the code meets the Project's requirement for BSD-2-Clause.
Request review: /cc @fallenpegasus /cc @hal.murray /cc @esr