Stop polluting global namespace

Add missing QT_BEGIN_NAMESPACE and QT_END_NAMESPACE to:
- barinstancing_p.h
- scatterinstancing_p.h
- qquickgraphstexturedata_p.h

Remove duplicate QT_BEGIN_NAMESPACE and QT_END_NAMESPACE from:
- qxyseries.h

Pick-to: 6.9 6.8
Fixes: QTBUG-138462
Change-Id: I51bd1881bf37009b9d896892f14724f1bcd2bad3
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 7f65202dc5)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/graphs2d/xychart/qxyseries.h

@@ -9,10 +9,6 @@
 
 QT_BEGIN_NAMESPACE
 class QModelIndex;
-QT_END_NAMESPACE
-
-QT_BEGIN_NAMESPACE
-
 class QXYSeriesPrivate;
 class QXYModelMapper;
 

src/graphs3d/engine/barinstancing_p.h

@@ -18,6 +18,8 @@
 
 #include <QtGui/qquaternion.h>
 
+QT_BEGIN_NAMESPACE
+
 struct BarItemHolder
 {
     QVector3D position = {.0f, .0f, .0f};
@@ -57,4 +59,6 @@ private:
     bool m_transparency = false;
 };
 
+QT_END_NAMESPACE
+
 #endif // BARINSTANCING_H

src/graphs3d/engine/scatterinstancing_p.h

@@ -19,6 +19,8 @@
 
 #include <QtGui/qquaternion.h>
 
+QT_BEGIN_NAMESPACE
+
 struct DataItemHolder
 {
     QVector3D position;
@@ -65,4 +67,6 @@ private:
     qsizetype m_previousHideIndex = -1;
 };
 
+QT_END_NAMESPACE
+
 #endif // SCATTERINSTANCING_H

src/graphs3d/qml/qquickgraphstexturedata_p.h

@@ -17,6 +17,8 @@
 #include <QList>
 #include <QtQuick3D/qquick3dtexturedata.h>
 
+QT_BEGIN_NAMESPACE
+
 class QQuickGraphsTextureData : public QQuick3DTextureData
 {
     Q_OBJECT
@@ -31,4 +33,6 @@ private:
     QColor linearInterpolate(QColor startColor, QColor endColor, float value);
 };
 
+QT_END_NAMESPACE
+
 #endif // QQUICKGRAPHSTEXTUREDATA_P_H