fix(turboquant): guard upstream-only grpc-server fields for fork (#10043)

fix(turboquant): guard upstream-only grpc-server fields for fork build

backend/cpp/llama-cpp/grpc-server.cpp is reused by the turboquant build,
which compiles against an older llama.cpp fork (TheTom/llama-cpp-turboquant).
Two recent changes added references to upstream-only struct fields outside the
existing LOCALAI_LEGACY_LLAMA_CPP_SPEC guards:

  - common_params::checkpoint_min_step (default + option handler), added with
    the ggml-org/llama.cpp 35c9b1f3 bump (#9998)
  - the common_params_speculative::draft tensor_buft_overrides sentinel
    termination (#9919), which sat after the guard's #endif

The fork has neither field, so grpc-server.cpp failed to compile for every
turboquant flavor. Wrap the three references in #ifndef
LOCALAI_LEGACY_LLAMA_CPP_SPEC, matching the existing fork-compat guards, so the
stock llama-cpp build is unchanged and the fork build skips them. Update
patch-grpc-server.sh's doc comment to record what the macro now gates out.

Verified by a local fallback-flavor turboquant build: grpc-server.cpp compiles
against the fork and the backend image builds.


Assisted-by: Claude:claude-opus-4-7 [Claude Code]

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>