Add missing #includes

This is needed by static analysis tools (which compile each header on
its own, IIUC), e.g. clang-tidy:
src/widgets/itemviews/qtableview_p.h:99:22: error: unknown type name
'QTableView' [clang-diagnostic-error].
    Q_DECLARE_PUBLIC(QTableView)
                     ^

Change-Id: I84a1221469e9a78f243cc892ef7c60da9b515718
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/corelib/itemmodels/qabstractproxymodel_p.h

@@ -16,6 +16,7 @@
 //
 //
 
+#include "qabstractproxymodel.h"
 #include "private/qabstractitemmodel_p.h"
 #include "private/qproperty_p.h"
 

src/corelib/itemmodels/qitemselectionmodel_p.h

@@ -15,6 +15,7 @@
 // We mean it.
 //
 
+#include "qitemselectionmodel.h"
 #include "private/qobject_p.h"
 #include "private/qproperty_p.h"
 

src/widgets/itemviews/qabstractitemview_p.h

@@ -16,6 +16,7 @@
 //
 
 #include <QtWidgets/private/qtwidgetsglobal_p.h>
+#include "qabstractitemview.h"
 #include "private/qabstractscrollarea_p.h"
 #include "private/qabstractitemmodel_p.h"
 #include "QtWidgets/qapplication.h"

src/widgets/itemviews/qheaderview_p.h

@@ -16,6 +16,7 @@
 //
 
 #include <QtWidgets/private/qtwidgetsglobal_p.h>
+#include "qheaderview.h"
 #include "private/qabstractitemview_p.h"
 
 #include "QtCore/qbitarray.h"

src/widgets/itemviews/qlistview_p.h

@@ -16,6 +16,7 @@
 //
 
 #include <QtWidgets/private/qtwidgetsglobal_p.h>
+#include "qlistview.h"
 #include "private/qabstractitemview_p.h"
 #include "qbitarray.h"
 #include "qbsptree_p.h"

src/widgets/itemviews/qtableview_p.h

@@ -16,6 +16,9 @@
 //
 
 #include <QtWidgets/private/qtwidgetsglobal_p.h>
+#include "qtableview.h"
+#include "qheaderview.h"
+
 #include <QtCore/QList>
 #include <QtCore/QMap>
 #include <QtCore/QSet>

src/widgets/itemviews/qwidgetitemdata_p.h

@@ -6,6 +6,7 @@
 
 #include <QtWidgets/private/qtwidgetsglobal_p.h>
 #include <QtCore/qdatastream.h>
+#include <QtCore/qvariant.h>
 
 //
 //  W A R N I N G