Compile with recent Qt6

Some implicit #include <QPointer> apparently vanished in qtbase with
a0d931e4822.

Change-Id: I5e6c821fb0a8997c1b5767aeb7bd4007df7d501f
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/solutions/tasking/tasktree.cpp

@@ -6,6 +6,7 @@
 #include <QEventLoop>
 #include <QFutureWatcher>
 #include <QPromise>
+#include <QPointer>
 #include <QSet>
 #include <QTimer>
 

src/plugins/coreplugin/progressmanager/processprogress.cpp

@@ -10,6 +10,7 @@
 #include <utils/qtcassert.h>
 
 #include <QFutureWatcher>
+#include <QPointer>
 
 using namespace Utils;
 

src/plugins/coreplugin/progressmanager/taskprogress.cpp

@@ -12,6 +12,7 @@
 #include <utils/qtcassert.h>
 
 #include <QFutureWatcher>
+#include <QPointer>
 #include <QTimer>
 
 using namespace Utils;

src/plugins/qmakeprojectmanager/qmakeparsernodes.h

@@ -17,6 +17,7 @@
 #include <QLoggingCategory>
 #include <QMap>
 #include <QPair>
+#include <QPointer>
 #include <QStringList>
 
 #include <memory>