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/1544808928-20002-3-git-send-email-jorge.ramirez-ortiz@linaro.orgSigned-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Himanshu Jha <himanshujha199640@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -468,6 +468,7 @@ our $logFunctions = qr{(?x:
 
 our $signature_tags = qr{(?xi:
 	Signed-off-by:|
+	Co-developed-by:|
 	Acked-by:|
 	Tested-by:|
 	Reviewed-by:|