chore(examples): fix unused-variable warning (#3732)

Co-authored-by: pengyiqiang <pengyiqiang@xiaomi.com>

examples/widgets/msgbox/lv_example_msgbox_1.c

@@ -4,6 +4,7 @@
 static void event_cb(lv_event_t * e)
 {
     lv_obj_t * obj = lv_event_get_current_target(e);
+    LV_UNUSED(obj);
     LV_LOG_USER("Button %s clicked", lv_msgbox_get_active_btn_text(obj));
 }