Ensure clipboard data is available after KiCad closes

Always Flush() after SetData()

pcbnew/board_stackup_manager/panel_board_stackup.cpp

@@ -299,6 +299,7 @@ void PANEL_SETUP_BOARD_STACKUP::onExportToClipboard( wxCommandEvent& event )
         // This data objects are held by the clipboard,
         // so do not delete them in the app.
         wxTheClipboard->SetData( new wxTextDataObject( report ) );
+        wxTheClipboard->Flush(); // Allow data to be available after closing KiCad
         wxTheClipboard->Close();
     }
 }