WindowX11 extra methods `setClassHint` & `setIconData` fails on (some?) Linux distro

On Guix linux distro (weird ones, so beware, the issue might be an idiosyncrasy from my distro) I run
`./script/run.py --jwm-version 0.4.18`

and got these 2 errors:

```
examples/dashboard/java/Example.java:89: error: cannot find symbol
((WindowX11) window).setClassHint("jwm-dashboard-example"); // allows OS-wide identification of the window (e.g. icon themes, .desktop files)
^
symbol: method setClassHint(String)
location: class WindowX11
examples/dashboard/java/Example.java:94: error: cannot find symbol
((WindowX11) window).setIconData(info.getWidth(), info.getHeight(), i.readPixels());
^
symbol: method setIconData(int,int,byte[])
location: class WindowX11
2 errors
Traceback (most recent call last):
```