[S] Change in coreboot[master]: checkpatch: add Co-authored-by to signature list

[Angel Pons (Code Review)]

Attention is currently required from: Felix Singer, Patrick Georgi, Martin L Roth, Paul Menzel, Tim Wawrzynczak, Michael Niewöhner.

Angel Pons has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/69166 )

Change subject: checkpatch: add Co-authored-by to signature list
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Patch Set 1: Code-Review+2

(1 comment)

Commit Message:

https://review.coreboot.org/c/coreboot/+/69166/comment/3761be9c_38d4982d 
PS1, Line 10: author. Add it to the list to make Jenkins happy.
> I guess it's just a more explicit way of clarifying authorship for the human reader. […]

Interesting, never seen `Co-authored-by` in use before. It can be useful in case someone doesn't want to sign off but still contributed to a change. Let's add it.