Qmake: Deactivate parse guard before it can cause trouble

... in the QmakeBuildSystem destructor

Change-Id: I23336cc0b72673fd2c63235b1c568439159d3d8d
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -231,6 +231,7 @@ QmakeBuildSystem::QmakeBuildSystem(QmakeBuildConfiguration *bc)
 
 QmakeBuildSystem::~QmakeBuildSystem()
 {
+    m_guard = {};
     delete m_cppCodeModelUpdater;
     m_cppCodeModelUpdater = nullptr;
     m_asyncUpdateState = ShuttingDown;