bpf, doc: add missing patchwork url and libbpf to maintainers

Add missing bits under tools/lib/bpf/ and also Q: entry in order to
make it easier for people to retrieve current patch queue.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>