declare el-collapse-transition in its dependents (#4728)

packages/collapse/src/collapse-item.vue

@@ -14,6 +14,7 @@
   </div>
 </template>
 <script>
+  import ElCollapseTransition from 'element-ui/src/transitions/collapse-transition';
   import Emitter from 'element-ui/src/mixins/emitter';
 
   export default {
@@ -23,6 +24,8 @@
 
     mixins: [Emitter],
 
+    components: { ElCollapseTransition },
+
     data() {
       return {
         contentWrapStyle: {