Check also the submember attributes during the LDAP group synchronization

- a group within our company LDAP contains members and submembers
  --> also the submember attributes need to be check for the LDAP group synchronization

lib/gitlab/ldap/group.rb

@@ -50,7 +50,9 @@ module Gitlab
       end
 
       def member_dns
-        if entry.respond_to? :member
+        if (entry.respond_to? :member) && (entry.respond_to? :submember)
+          entry.member + entry.submember
+        elsif entry.respond_to? :member
           entry.member
         elsif entry.respond_to? :uniquemember
           entry.uniquemember