ffmpeg-v4l2-request-fourier (debian): drop --enable-libxml2 (runner SONAME skew)

The Gitea debian-aarch64 runner has been upgraded past Debian trixie
and now ships libxml2 ≥ 2.14 (SONAME 16) while higgs (and any other
trixie target) still has libxml2 2.12 (SONAME 2).  -5 built cleanly,
but on higgs the daedalus-v4l2 daemon's dlopen of libavformat.so.62
fails:

    dlopen(libavformat.so.62): libxml2.so.16:
    cannot open shared object file: No such file or directory

Drop --enable-libxml2 from the Debian configure invocation; remove
the libxml2 entry from Depends; remove libxml2-dev from the CI
build-deps.  FFmpeg's libxml2-backed DASH demuxer is unused on the
Fourier fleet — daedalus-v4l2 daemon feeds AVPackets straight to
avcodec_send_packet (no demux); mpv-fourier uses ytdlp + mpv's own
stream code; firefox-fourier uses gecko-media's DASH demux.

Bumps PKGREL 5 → 6.  No source code or substitution-patch change.
Mirrors the libva trixie/runner ABI-skew workaround pattern
(marfrit-packages PR #62).

Arch PKGBUILD unaffected — Arch runner + Arch consumers both
rolling, libxml2 SONAMEs match.

After this lands, re-deploy on higgs via:
    sudo apt update && sudo apt install -y ffmpeg-v4l2-request-fourier
    sudo systemctl restart daedalus-v4l2

.gitea/workflows/build.yml

@@ -935,7 +935,7 @@ jobs:
               libfontconfig-dev libfribidi-dev libgmp-dev libgnutls28-dev \
               libmp3lame-dev libass-dev libdav1d-dev libdrm-dev \
               libfreetype-dev libpulse-dev libva-dev libvorbis-dev libvpx-dev \
-              libwebp-dev libx264-dev libx265-dev libxml2-dev libopus-dev \
+              libwebp-dev libx264-dev libx265-dev libopus-dev \
               libvulkan-dev glslang-tools \
               v4l-utils liblzma-dev zlib1g-dev \
               curl ca-certificates openssh-client rsync dpkg-dev