ColorPicker: Fixed `Unknown custom element: <el-button>` error

packages/color-picker/src/components/picker-dropdown.vue

@@ -43,6 +43,7 @@
   import Popper from 'element-ui/src/utils/vue-popper';
   import Locale from 'element-ui/src/mixins/locale';
   import ElInput from 'element-ui/packages/input';
+  import ElButton from 'element-ui/packages/button';
 
   export default {
     name: 'el-color-picker-dropdown',
@@ -53,7 +54,8 @@
       SvPanel,
       HueSlider,
       AlphaSlider,
-      ElInput
+      ElInput,
+      ElButton
     },
 
     props: {