tools/bpftool: ignore build products

For untracked things of tools/bpf, add this.

Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>