tools: bpftool: Cleanup license mess

Precise and non-ambiguous license information is important. The recent
relicensing of the bpftools introduced a license conflict.

The files have now:

     SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause

and

     * This program is free software; you can redistribute it and/or
     * modify it under the terms of the GNU General Public License
     * as published by the Free Software Foundation; either version
     * 2 of the License, or (at your option) any later version

Amazingly about 20 people acked that change and neither they nor the
committer noticed. Oh well.

Digging deeper: The files were imported from the iproute2 repository with
the GPL V2 or later boiler plate text in commit b66e907c ("tools:
bpftool: copy JSON writer from iproute2 repository")

Looking at the iproute2 repository at

  git://git.kernel.org/pub/scm/network/iproute2/iproute2.git

the following commit is the equivivalent:

  commit d9d8c839 ("json_writer: add SPDX Identifier (GPL...