distcheck fails because of .intltool-merge-cache.lock
Job [#26026](https://gitlab.xfce.org/panel-plugins/xfce4-windowck-plugin/-/jobs/26026) failed for 7c0602274c93a49d16296e8c91fc96e72f5ff775:
```
ERROR: files left in build directory after distclean:
./po/.intltool-merge-cache.lock
```
issue