selftests/bpf: Add missing files required by test_bpftool.sh for installing

test_bpftool.sh relies on bpftool and test_bpftool.py.

'make install' will install bpftool to INSTALL_PATH/bpf/bpftool, and
export it to PATH so that it can be used after installing.

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210820015556.23276-5-lizhijian@cn.fujitsu.com