Backends: GLFW: Fixed unused variable warning for empty assert macro. (#4459)

backends/imgui_impl_glfw.cpp

@@ -913,6 +913,7 @@ static int ImGui_ImplGlfw_CreateVkSurface(ImGuiViewport* viewport, ImU64 vk_inst
 {
     ImGui_ImplGlfw_Data* bd = ImGui_ImplGlfw_GetBackendData();
     ImGui_ImplGlfw_ViewportData* vd = (ImGui_ImplGlfw_ViewportData*)viewport->PlatformUserData;
+    IM_UNUSED(bd);
     IM_ASSERT(bd->ClientApi == GlfwClientApi_Vulkan);
     VkResult err = glfwCreateWindowSurface((VkInstance)vk_instance, vd->Window, (const VkAllocationCallbacks*)vk_allocator, (VkSurfaceKHR*)out_vk_surface);
     return (int)err;