utils: checkstyle.py: Add Co-developed-by to known_trailers

Add Co-developed-by to known_trailers to silence false positive
"Invalid commit trailer key 'Co-developed-by'" warnings.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

utils/checkstyle.py

@@ -478,6 +478,7 @@ class TrailersChecker(CommitChecker):
         'Signed-off-by': email_regex,
         'Suggested-by': email_regex,
         'Tested-by': email_regex,
+        'Co-developed-by': email_regex,
     }
 
     trailer_regex = re.compile(r'([A-Z][a-zA-Z-]*)\s*:\s*(.*)')