Bug#534063: pixfrogger: FTBFS: Nonexistent build-dependency: fenix

[Jeremy Bicha]

According to codesearch, this is the only place the XS field is used in Debian:
https://codesearch.debian.net/results/%20path%3Adebian%2Fcontrol%20XS-Build-Indep-Architecture

and therefore triggers
https://lintian.debian.org/tags/unknown-field-in-dsc.html

piuparts complains too:
https://piuparts.debian.org/sid/source/p/pixfrogger.html

Thanks,
Jeremy Bicha