bpftool: Declare generator name

Most code generators declare its name so did this for bfptool.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220509090247.5457-1-jasowang@redhat.com