chore: fix warning when log disable (#5018)

Signed-off-by: FASTSHIFT <vifextech@foxmail.com>

examples/others/observer/lv_example_observer_2.c

@@ -30,6 +30,7 @@ static void engine_state_observer_cb(lv_observer_t * observer, lv_subject_t * su
     LV_UNUSED(observer);
 
     int32_t v = lv_subject_get_int(subject);
+    LV_UNUSED(v);
     /*In a real application set/clear a pin here*/
     LV_LOG_USER("Engine state: %d", v);
 }