tools: bpftool: add out edges for each basic-block

This patch adds out edges for each basic-block. We will need these out
edges to finish the .dot graph drawing.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>