Fix unused variable warning in ImGui::EndListBox() (#3897)

imgui_widgets.cpp

@@ -6220,6 +6220,7 @@ void ImGui::EndListBox()
     ImGuiContext& g = *GImGui;
     ImGuiWindow* window = g.CurrentWindow;
     IM_ASSERT((window->Flags & ImGuiWindowFlags_ChildWindow) && "Mismatched BeginListBox/EndListBox calls. Did you test the return value of BeginListBox?");
+    IM_UNUSED(window);
 
     EndChildFrame();
     EndGroup(); // This is only required to be able to do IsItemXXX query on the whole ListBox including label