format-patch -s shouldnt sign 2nd time

From: jim.cromie@gmail.com <jim.cromie@gmail.com>
Date: Oct 8, 2021, 06:50 UTC
Message-ID: <CAJfuBxwGJmJvH-=Cm=6ZXNzHxWUL=TKjV6JRqPpVDB9xuiPDeQ@mail.gmail.com>

Ive started to manually add SOB since it needs to be above the ---, followed by version info.

git format-patch -s ... will sign again, and then checkpatch will complain

WARNING: Duplicate signature
#178:
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>

A selective addition would simplify this situation.