CppEditor: Make sure fallback project part is up to date

... with regards to the session include paths.
Amends 0636238429.

Fixes: QTCREATORBUG-26323
Change-Id: I8e2cd5f5e87d9dc3d2df3f943e13599bc7139768
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/cppmodelmanager.cpp

@@ -694,6 +694,10 @@ CppModelManager::CppModelManager()
 
     connect(KitManager::instance(), &KitManager::kitsChanged, this,
             &CppModelManager::setupFallbackProjectPart);
+    connect(this, &CppModelManager::projectPartsRemoved, this,
+            &CppModelManager::setupFallbackProjectPart);
+    connect(this, &CppModelManager::projectPartsUpdated, this,
+            &CppModelManager::setupFallbackProjectPart);
     setupFallbackProjectPart();
 
     qRegisterMetaType<CPlusPlus::Document::Ptr>("CPlusPlus::Document::Ptr");