Qt6: fully define dive structure

With Qt the forward declaration fails as the export to QML for the statistics requires
the MOC code to be able to determine the sizeof(struct dive).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

stats/statshelper.h

@@ -8,8 +8,7 @@
 #include <vector>
 #include <QPointF>
 #include <QSGNode>
-
-struct dive;
+#include "core/dive.h"
 
 // Round positions to integer values to avoid ugly artifacts
 QPointF roundPos(const QPointF &p);