forklift: Add signature to forklifted patches

Add a signature to the forklifted patches to more easily track patches
which were landed using this tool.

Change-Id: I906e92da4e639b56d7ed72995861d147c5b6cd74
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3275739
Reviewed-by: Drew Davenport <ddavenport@chromium.org>
Tested-by: Sean Paul <seanpaul@chromium.org>
Commit-Queue: Sean Paul <seanpaul@chromium.org>