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.cpp

@@ -100,7 +100,6 @@
 #include "statistics.h"
 #include "torrentimpl.h"
 #include "tracker.h"
-#include "trackerentry.h"
 
 static const char PEER_ID[] = "qB";
 static const char RESUME_FOLDER[] = "BT_backup";