Silence new stylelint complaint (#3197)
Cf. https://github.com/FreshRSS/FreshRSS/pull/3196#issuecomment-699928812
```
p/themes/Ansum/_layout.scss
42:4 ✖ Unexpected empty line before rule rule-empty-line-before
p/themes/Mapco/_layout.scss
42:4 ✖ Unexpected empty line before rule rule-empty-line-before
```