build-aux: templates: make build reproducible

By including @filename@ in the enums/errors types header template, we generate files containing the full path to source files, which reflects the particular filesystem layout used for a given build. As such, this alters build reproducibility as it causes a (admittedly small) diff between builds executed in different environments.

Using @basename@ instead ensures only the file name (without its full path) is mentioned in the generated files, making the builds reproducible again.

This should be backported to the mm-1-20 branch as well, if possible