Silence some compiler warnings

This is what clang suggested in compiler warnings.

Signed-off-by: Robert C. Helling <helling@atdotde.de>

stats/barseries.h

@@ -13,8 +13,8 @@
 #include <QGraphicsRectItem>
 
 class QGraphicsScene;
-class InformationBox;
-class StatsVariable;
+struct InformationBox;
+struct StatsVariable;
 
 class BarSeries : public StatsSeries {
 public:

stats/boxseries.h

@@ -13,7 +13,7 @@
 #include <QGraphicsLineItem>
 #include <QGraphicsRectItem>
 
-class InformationBox;
+struct InformationBox;
 class QGraphicsScene;
 
 class BoxSeries : public StatsSeries {

stats/pieseries.cpp

@@ -116,7 +116,7 @@ PieSeries::PieSeries(QGraphicsScene *scene, StatsAxis *xAxis, StatsAxis *yAxis,
 		  { return std::make_tuple(-data[idx1].second, idx1) <
 			   std::make_tuple(-data[idx2].second, idx2); });
 	auto it = std::find_if(sorted.begin(), sorted.end(),
-			       [count=totalCount, &data, smallest_slice_percentage](int idx)
+			       [count=totalCount, &data](int idx)
 			       { return data[idx].second * 100 / count < smallest_slice_percentage; });
 	if (it - sorted.begin() < min_slices) {
 		// Take minimum amount of slices below 50%...

stats/pieseries.h

@@ -9,7 +9,7 @@
 #include <vector>
 #include <QString>
 
-class InformationBox;
+struct InformationBox;
 class QGraphicsEllipseItem;
 class QGraphicsScene;
 class QGraphicsSimpleTextItem;

stats/scatterseries.h

@@ -12,7 +12,7 @@
 
 class QGraphicsPixmapItem;
 class QGraphicsScene;
-class InformationBox;
+struct InformationBox;
 struct StatsVariable;
 struct dive;