Move qt6-multimedia-ffmpeg to kde-dependencies

We only have it because of a few pre-installed KDE apps; make that clear
by moving it to the semantically correct place.

mkosi.conf.d/00-packages-core.conf

@@ -22,7 +22,6 @@ Packages=
     noto-fonts-cjk
     noto-fonts-emoji
     plymouth
-    qt6-multimedia-ffmpeg
     realtime-privileges # `realtime` group and realtime privileges for processes using it
     socat
     systemd

mkosi.conf.d/45-packages-kde-dependencies.conf

@@ -37,6 +37,7 @@ Packages=
     mesa-utils # Provides both eglinfo and glxinfo, used by kinfocenter
     opencv # spectacle
     openexr # kimageformats
+    qt6-multimedia-ffmpeg # ffmpeg backend for qt6-multimedia, used in Dolphin, Konsole, Spectacle, and Prison
     rsync # kup, for one of the backends
     switcheroo-control # kio, for launching apps on non-default GPU
     tesseract-data-eng # Tesseract requires at least one language backend