Qt/debugger: Don't update the symbols list unnecessarily

It only needs to be updated when we changes the symbols, not every time the code widget updates and it does take a while to update them so this fixes some delay when updating the code window.

Source/Core/DolphinQt2/MenuBar.h

@@ -102,6 +102,9 @@ signals:
   void RecordingStatusChanged(bool recording);
   void ReadOnlyModeChanged(bool read_only);
 
+  // Synbols
+  void NotifySymbolsUpdated();
+
 private:
   void OnEmulationStateChanged(Core::State state);