Add root to conf.paths to fix less parse error when absolute paths imported

当在`less`文件中`import`一个以绝对路径表示的文件时，`less`报错：
```
[ERROR] parser.less: '/Users/lee/dev/work/sglobal/_build/fis2/mixin.less' wasn't found [/Users/lee/dev/work/sglobal/widget/dialog/dialog.less:1:0]
```

在`paths`中增加了根目录以支持`import`绝对路径。