fix Qt4 build failure

src/Gui/Placement.cpp

@@ -140,6 +140,7 @@ void Placement::showDefaultButtons(bool ok)
     ui->closeButton->setVisible(ok);
     ui->applyButton->setVisible(ok);
     ui->buttonBoxLayout->invalidate();
+#if QT_VERSION >= 0x050000
     if (ok) {
         //ui->buttonBoxSpacer->changeSize(88, 24, QSizePolicy::Expanding, QSizePolicy::Minimum);
         ui->buttonBoxLayout->insertItem(0, ui->buttonBoxSpacer);
@@ -148,6 +149,7 @@ void Placement::showDefaultButtons(bool ok)
         //ui->buttonBoxSpacer->changeSize(0, 0, QSizePolicy::Ignored, QSizePolicy::Ignored);
         ui->buttonBoxLayout->removeItem(ui->buttonBoxSpacer);
     }
+#endif
 }
 
 void Placement::open()