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

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

examples/widgets/checkbox/lv_example_checkbox_1.c

@@ -5,9 +5,12 @@ static void event_handler(lv_event_t * e)
 {
     lv_event_code_t code = lv_event_get_code(e);
     lv_obj_t * obj = lv_event_get_target(e);
+    LV_UNUSED(obj);
     if(code == LV_EVENT_VALUE_CHANGED) {
         const char * txt = lv_checkbox_get_text(obj);
         const char * state = lv_obj_get_state(obj) & LV_STATE_CHECKED ? "Checked" : "Unchecked";
+        LV_UNUSED(txt);
+        LV_UNUSED(state);
         LV_LOG_USER("%s: %s", txt, state);
     }
 }