audio/mumble-server: Fix "Failed to set TOS for UDP Socket" for IPv6

<W>2025-12-29 23:37:57.715 1 => Server listening on [::]:64738
<W>2025-12-29 23:37:57.721 1 => Server: Failed to set TOS for UDP Socket

This prevents proper QoS from being set for IPv6.

Upstream issue: https://github.com/mumble-voip/mumble/issues/7088

While here pet portclippy.

PR: 292034
MFH: 2026Q1