[TD] add missing update when changing WeldSymbol parameter

see https://forum.freecadweb.org/viewtopic.php?f=35&t=40608&p=364690#p364690

src/Mod/TechDraw/Gui/ViewProviderWeld.cpp

@@ -94,6 +94,16 @@ void ViewProviderWeld::updateData(const App::Property* prop)
     ViewProviderDrawingView::updateData(prop);
 }
 
+void ViewProviderWeld::onChanged(const App::Property* p)
+{
+    QGIView* qgiv = getQView();
+    if (qgiv) {
+        qgiv->updateView(true);
+    }
+
+    ViewProviderDrawingView::onChanged(p);
+}
+
 std::vector<App::DocumentObject*> ViewProviderWeld::claimChildren(void) const
 {
     // Collect any child Document Objects and put them in the right place in the Feature tree

src/Mod/TechDraw/Gui/ViewProviderWeld.h

@@ -55,6 +55,7 @@ public:
     /// returns a list of all possible modes
     virtual std::vector<std::string> getDisplayModes(void) const;
     virtual void updateData(const App::Property*);
+    virtual void onChanged(const App::Property* p);
     virtual std::vector<App::DocumentObject*> claimChildren(void) const;
     virtual bool setEdit(int ModNum);
     virtual void unsetEdit(int ModNum);