Since #28732 our internal symbols are exported. A duplicate copy dlopened and
dlclosed by ggml_backend_load_all() then interposes them, so its destructors
destroy the live vk_instance and later device queries hit the GGML_ASSERT on
vk_instance.device_indices. Hidden visibility exports only GGML_BACKEND_API,
as before #28732.
Fixes#29138
Assisted-by: henk:claude-fable-5