C++: Fix resolving ui_* files in CppPreprocessor

The working copy contains the artificial ui_* files and therefore we
have to consider it while resolving include files.

Task-number: QTCREATORBUG-9683
Change-Id: Icb3387b4cd885b3652bae3f1da465d3e0f633332
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

tests/cppmodelmanager/testdata_guiproject1/mainwindow.h

@@ -0,0 +1,24 @@
+// Copyright license
+
+#ifndef MAINWINDOW_H
+#define MAINWINDOW_H
+
+#include <QMainWindow>
+
+namespace Ui {
+class MainWindow;
+}
+
+class MainWindow : public QMainWindow
+{
+    Q_OBJECT
+
+public:
+    explicit MainWindow(QWidget *parent = 0);
+    ~MainWindow();
+
+private:
+    Ui::MainWindow *ui;
+};
+
+#endif // MAINWINDOW_H