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

diff --git a/imgui_widgets.cpp b/imgui_widgets.cpp
index 9db106e..0d4a2d4 100644
--- a/imgui_widgets.cpp
+++ b/imgui_widgets.cpp
@@ -6220,6 +6220,7 @@
     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