scripts: checkpatch: add Co-authored-by tag

Add Co-authored-by tag which is mainly used by Github.

Signed-off-by: Bartosz Bilas <bartosz.bilas@hotmail.com>
diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 7b39583..137e503 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -493,6 +493,7 @@
 
 our $signature_tags = qr{(?xi:
 	Signed-off-by:|
+	Co-authored-by:|
 	Co-developed-by:|
 	Acked-by:|
 	Tested-by:|