checkpatch: add Co-developed-by to signature tags

checkpatch: add Co-developed-by to signature tags

As per Documentation/process/submitting-patches, Co-developed-by is a
valid signature.

This commit removes the warning.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jorge Ramirez-Ortiz <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Himanshu Jha <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Niklas Cassel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>