libbpf: relicense bpf_helpers.h and bpf_endian.h

bpf_helpers.h and bpf_endian.h contain useful macros and BPF helper
definitions essential to almost every BPF program. Which makes them
useful not just for selftests. To be able to expose them as part of
libbpf, though, we need them to be dual-licensed as LGPL-2.1 OR
BSD-2-Clause. This patch updates licensing of those two files.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Hechao Li <hechaol@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Lawrence Brakmo <brakmo@fb.com>
Acked-by: Adam Barth <arb@fb.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Josef Bacik <jbacik@fb.com>
Acked-by: Joe Stringer <joe@wand.net.nz>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: David Ahern <dsahern@gmail.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Lorenz Bauer <lmb@cloudflare.com>
Acked-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Acked-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Petar Penkov <ppenkov@google.com>
Acked-by: Teng Qin <palmtenor@gmail.com>
Cc: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Michal Rostecki <mrostecki@opensuse.org>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Sargun Dhillon <sargun@sargun.me>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>