checkpatch: add Co-authored-by to signature list

Co-authored-by is commonly used for changes that have more than one
author. Add it to the list to make Jenkins happy.

Change-Id: I7f66824febe3be756c64ebf44c94bc653a66f1e1
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69166
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Georgi <patrick@coreboot.org>

util/lint/checkpatch.pl

@@ -618,6 +618,7 @@ our $signature_tags = qr{(?xi:
 	Reviewed-by:|
 	Reported-by:|
 	Suggested-by:|
+	Co-authored-by:|
 	To:|
 	Cc:
 )};