fix(libcamera): drop MergePolicy arg for Debian Trixie libcamera ABI

[issic]

Summary

On Debian 13 (Trixie), the system-packaged libcamera does not expose the
two-argument ControlList::merge(other, MergePolicy) overload, so
device/libcamera/buffer.cc fails to compile.

This patch drops the explicit MergePolicy::OverwriteExisting argument,
falling back to the single-argument form which is present in both the
Trixie-packaged libcamera and in the rpt-suffixed Raspberry Pi builds.
OverwriteExisting is the default merge behavior in the single-argument
overload, so semantics are preserved on platforms where the two-argument
form is available.

Tested on

• Debian 13 (Trixie), aarch64, BTT CB1 on Manta M4P
• Camera: Raspberry Pi Camera Module via libcamera
• Build: clean, runtime: streams at 1280x720 / 15fps with no regression

Notes

• Affects only the libcamera device backend (CSI cameras); USB / V4L2 path
  is unchanged.
• One-line ABI-compat fix; underlying merge semantics are unchanged.
• Related to broader libcamera version compatibility (new libcamera 0.2.0+rpt20240215-1 makes precompiled (0.2.8) camera-streamer.service failing #137, libcamera not supported #156).

[mryel00]

Iirc there was an issue if MergePolicy::OverwriteExisting wasn't set.
I explicitely added this commit back when I fixed #206: a9f4939
edit: I have to check it myself, as I don't know it out of my head

[mryel00]

It's still compiling and I don't see any issue immediately.
I might have set it to copy the old behavior of moving the controls.