utils: checkstyle.py: Accept Closes commit trailer

`Closes: <url>` is recognized by gitlab[0] and closes the referenced issue
automatically when a commit is made to the project's default branch. Now
that issues are hosted on gitlab, it will be the preferred way to reference
the fixed issue from a commit, so replace the previously used `Bug` with it
in the checkstlye.py script.

[0]: https://docs.gitlab.com/user/project/issues/managing_issues/#default-closing-pattern

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>

utils/checkstyle.py

@@ -541,7 +541,7 @@ class TrailersChecker(CommitChecker):
 
     known_trailers = {
         'Acked-by': email_regex,
-        'Bug': link_regex,
+        'Closes': link_regex,
         'Co-developed-by': email_regex,
         'Fixes': commit_regex,
         'Link': link_regex,