"bitbake -g m4-native -c do_unpack" doesn't list any dependencies in
task-depends.dot. This is because no header describing the task was
added unless a task has dependencies. Tweak the code to fix this.
[YOCTO #10893]
(Bitbake rev: e2fbe728769f09ef4178e2ef4d69a2f60ffe3ebb)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>