whisper.cpp: enable the ggml OpenVINO backend

Runs the whole model through OpenVINO, next to the Vulkan backend.
Both register as GPU devices and whisper uses one per run, selected
with whisper-cli -dev N, so the two can be compared on the same
build. whisper-bench has no device option; the README points at
whisper-cli's exit timings instead.

The backend includes opencl.hpp directly, so like openvino it needs
OpenCL-CLHPP 2026.05.29, which the buildsystem provides via a hint.
Test-built on -current, sbopkglint clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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