Mask Eris main() to fix windows builds (CB #5050 / GH #6690)

Co-authored-by: Benedikt Straub <benedikt-straub@web.de>
Co-authored-by: hessenfarmer <43039887+hessenfarmer@users.noreply.github.com>
Co-authored-by: hessenfarmer <stephan-lutz@gmx.de>
Co-authored-by: MarkMcWire <markmcwire@noreply.codeberg.org>
Co-authored-by: The Widelands Bunny Bot <bunnybot@noreply.codeberg.org>
Co-authored-by: The Widelands Translations Bot <bunnybot@widelands.org>
Co-authored-by: Tóth András <txa-dev@posteo.hu>
Co-authored-by: Widelands Bunnybot <bunnybot@widelands.org>

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