tracing/fastboot: fix bootgraph.pl to run with "use strict"

As a perl novice, I would prefer to have the benefit of the interpreters'
wisdom.  It turns out there were already some warnings, so let's fix them.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Acked-by: Frédéric Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>