Use complete types in container declarations

Qt 6 disallows to use incomplete types in container declarations
in some cases, e.g. in parameters of signals/slots.

src/base/bittorrent/session.h

@@ -50,6 +50,7 @@
 #include "cachestatus.h"
 #include "sessionstatus.h"
 #include "torrentinfo.h"
+#include "trackerentry.h"
 
 class QFile;
 class QNetworkConfiguration;
@@ -100,7 +101,6 @@ namespace BitTorrent
     class TorrentImpl;
     class Tracker;
     struct LoadTorrentParams;
-    struct TrackerEntry;
 
     enum class MoveStorageMode;