QbsProjectManager: Re-parse project when build system files change.

This was accidentally broken in fdc552bda6 so that only changes to the
top-level project file would trigger re-parsing.

Change-Id: Iab2d6b0d3187f5875fec916fe4dccbef08b54669
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

src/plugins/qbsprojectmanager/qbsproject.cpp

@@ -451,7 +451,7 @@ void QbsProject::updateAfterParse()
 {
     qCDebug(qbsPmLog) << "Updating data after parse";
     rootProjectNode()->update();
-    updateDocuments(QSet<QString>() << projectFilePath().toString());
+    updateDocuments(m_qbsProject.buildSystemFiles());
     updateBuildTargetData();
     updateCppCodeModel();
     updateQmlJsCodeModel();