Add missing "announce" channel type

Of note, this doesn't mean the channels will display, but it does fix
parsing errors which cause the whole chat display to fail.

osu.Game/Online/Chat/ChannelType.cs

@@ -13,5 +13,6 @@ namespace osu.Game.Online.Chat
         PM,
         Group,
         System,
+        Announce,
     }
 }