bpftool: Fix SPDX tag for Makefiles and .gitignore

Bpftool is dual-licensed under GPLv2 and BSD-2-Clause. In commit
907b2236

 ("tools: bpftool: dual license all files") we made sure
that all its source files were indeed covered by the two licenses, and
that they had the correct SPDX tags.

However, bpftool's Makefile, the Makefile for its documentation, and the
.gitignore file were skipped at the time (their GPL-2.0-only tag was
added later). Let's update the tags.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Joe Stringer <joe@cilium.io>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/bpf/20211105221904.3536-1-quentin@isovalent.com