metal: fix tuning include path in subprojects

[obrr-hhx]

Overview

Fixes #28114.

Resolve the Metal tuning include directory relative to its own CMakeLists.txt. When llama.cpp is included through FetchContent, CMAKE_SOURCE_DIR points to the consuming project and ggml-metal-tuning fails to find ggml-metal-tuning.h.

Validation

• Reproduced the missing-header build failure with a minimal FetchContent consumer before the change.
• Built and linked ggml-metal-tuning successfully in the same consumer after the change.
• Built and linked ggml-metal-tuning in a standalone Metal build.
• Passed applicable pre-commit-hooks v4.6.0 checks (trailing whitespace, final newline, file size) and git diff --check.

Requirements

The change is limited to the include path in tools/tuning/CMakeLists.txt. No runtime code changes.

[ggml-gh-bot]

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