audio/plasma6-kpipewire: Fix build with FFmpeg 8

Backport upstream commit 6dc69b0674f9946641e88151c209ce58f085d06a
to fix build with FFmpeg 8

Reference:
https://invent.kde.org/plasma/kpipewire/-/commit/6dc69b0674f9946641e88151c209ce58f085d06a

PR: 289481
Approved by: blanket, build fix