veristat: relicense veristat.c as dual GPL-2.0-only or BSD-2-Clause licensed

Dual-license veristat.c to dual GPL-2.0-only or BSD-2-Clause license.
This is needed to mirror it to Github to make it convenient for distro
packagers to package veristat as a separate package.

Veristat grew into a useful tool by itself, and there are already
a bunch of users relying on veristat as generic BPF loading and
verification helper tool. So making it easy to packagers by providing
Github mirror just like we do for bpftool and libbpf is the next step to
get veristat into the hands of users.

Apart from few typo fixes, I'm the sole contributor to veristat.c so
far, so no extra Acks should be needed for relicensing.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230331222405.3468634-2-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/veristat.c

-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
 /* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
 #define _GNU_SOURCE
 #include <argp.h>