Fix build

Otherwise, the build fails with the following error:

.../src/src/interface/Node_select.cpp:254:46: error: missing template arguments before ‘(’ token
  254 |             reassign_children_rec(*it, QFlags(Qt::Unchecked));
      |                                              ^

(at least on Debian with Qt 5.15.2)

src/interface/Node_select.cpp

@@ -253,7 +253,7 @@ void Node_select::on_reset_button_clicked(){
         if((*it)->parent()==nullptr){//we only want parent nodes
             _displayed_containers.push_back(((*it)->data(0,Qt::UserRole)).value<Container*>());
 #if defined(HAVE_QT5_15)
-            reassign_children_rec(*it, QFlags(Qt::Unchecked));
+            reassign_children_rec(*it, QFlags<Qt::CheckState>(Qt::Unchecked));
 #else
             reassign_children_rec(*it, 0);
 #endif