<LoginItem> has noProperty 'placeholder' (#1433)

add placeholder?: string;

src/components/Login/index.d.ts

@@ -18,6 +18,7 @@ export interface LoginItemProps {
   rules?: any[];
   style?: React.CSSProperties;
   onGetCaptcha?: () => void;
+  placeholder?: string;
 }
 
 export class LoginItem extends React.Component<LoginItemProps, any> {}