Fix button icon margin style bug

```jsx
<Button loading>
  <TwoChineseWords />
</Button>
```

components/button/style/index.less

@@ -150,7 +150,7 @@
     letter-spacing: .34em;
   }
 
-  &-two-chinese-chars > * {
+  &-two-chinese-chars > *:not(.@{iconfont-css-prefix}) {
     letter-spacing: .34em;
     margin-right: -.34em;
   }