New review comment by classabbyamp on void-packages repository

https://github.com/void-linux/void-packages/pull/34560#discussion_r780717805

Comment:
```suggestion
license="GPL-3.0-only"
```
from [here](https://spdx.org/licenses/GPL-3.0-only.html)/[Manual.md](https://github.com/void-linux/void-packages/blob/master/Manual.md#var_license)