buildsteplist.cpp: Fix compile error for Qt < 5.13

QList::swapItemsAt() was added in Qt 5.13 (without \since).

Amends 963dc84cc5.

Change-Id: I40e4a5eda3540ae4a3212ea44413ed8a6cf17ce4
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/projectexplorer/buildsteplist.cpp

@@ -185,7 +185,11 @@ bool BuildStepList::removeStep(int position)
 
 void BuildStepList::moveStepUp(int position)
 {
+#if QT_VERSION >= QT_VERSION_CHECK(5, 13, 0)
     m_steps.swapItemsAt(position - 1, position);
+#else
+    m_steps.swap(position - 1, position);
+#endif
     emit stepMoved(position, position - 1);
 }