Fix unused variable warning in ImGui::EndListBox()

[fountainment]

This commit fixed an unused variable warning in function ImGui::EndListBox().

Compiler log:

[  0%] Building CXX object External/imgui/CMakeFiles/imgui.dir/imgui_widgets.cpp.o
/home/even/Projects/cherrysoda-engine/External/imgui/imgui_widgets.cpp: In function ‘void ImGui::EndListBox()’:
/home/even/Projects/cherrysoda-engine/External/imgui/imgui_widgets.cpp:6198:18: warning: unused variable ‘window’ [-Wunused-variable]
 6198 |     ImGuiWindow* window = g.CurrentWindow;
      |                  ^~~~~~
[100%] Linking CXX static library ../../lib/libimgui.a

Compiler version: gcc (GCC) 10.2.0
Platform: Linux 5.11.2-1-MANJARO x86_64

[ocornut]

Thank you. Please note this only happens if your IM_ASSERT macro is defined as empty.

We are going to add a CI task around the existing "extra warnings" tasks to build with IM_ASSERT disabled to increase the visibility of those.