musa: add GGML_UNUSED_VARS

[yeahdongcn]

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This PR introduces a new GGML_UNUSED_VARS macro to handle multiple unused variable declarations, replacing verbose individual GGML_UNUSED() calls throughout the codebase.

The current approach of marking unused variables requires multiple lines:

GGML_UNUSED(var1);
GGML_UNUSED(var2);
GGML_UNUSED(var3);

This creates visual clutter and is harder to maintain when adding/removing variables from the unused list.

Testing Done

• MUSA build passed with no warning.

  cmake -B build -DGGML_MUSA=ON
  cmake --build build -j $(nproc) --config Release

• docker build -t local/llama.cpp:server-cuda --target server -f .devops/cuda.Dockerfile .