msbuild: add pch for dolphinqt

since this now includes Qt headers it's better than only using
the vanilla DolphinLib one alone (previous behavior before Qt6)

Source/Core/DolphinQt/pch_qt.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include "pch.h"
+
+#include <QComboBox>
+#include <QGridLayout>
+#include <QHeaderView>
+#include <QLabel>
+#include <QList>
+#include <QListWidget>
+#include <QObject>
+#include <QString>
+#include <QTableWidget>
+#include <QWidget>