bpftool: exclude bash-completion/bpftool from .gitignore pattern

tools/bpf/bpftool/.gitignore has the "bpftool" pattern, which is
intended to ignore the following build artifact:

  tools/bpf/bpftool/bpftool

However, the .gitignore entry is effective not only for the current
directory, but also for any sub-directories.

So, from the point of .gitignore grammar, the following check-in file
is also considered to be ignored:

  tools/bpf/bpftool/bash-completion/bpftool

As the manual gitignore(5) says "Files already tracked by Git are not
affected", this is not a problem as far as Git is concerned.

However, Git is not the only program that parses .gitignore because
.gitignore is useful to distinguish build artifacts from source files.

For example, tar(1) supports the --exclude-vcs-ignore option. As of
writing, this option does not work perfectly, but it intends to create
a tarball excluding files specified by .gitignore.

So, I believe it is better to fix this issue.

You can fix it by prefixing the pattern with a slash; the leading slash
means the specified pattern is relative to the current directory.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>