ggml-cpu : prevent kleidiai build/install targets

[anyj0527]

• currently cmake install targets of kleidiai generate error
• set SOURCE_SUBDIR to explict invalid path as a bypass
• Add option EXCLUDE_FROM_ALL will do the same fix but it requires camke > v3.28.

REF: https://discourse.cmake.org/t/prevent-fetchcontent-makeavailable-to-execute-cmakelists-txt/12704/3

Current install error log:

$ cmake --install build-kai --prefix install-kai
-- Install configuration: "Release"
-- Installing: /home/yongjoo/llama.cpp/install-kai/lib/libggml-cpu.so
CMake Error at build-kai/_deps/kleidiai_download-build/cmake_install.cmake:52 (file):
  file INSTALL cannot find
  "/home/yongjoo/llama.cpp/build-kai/bin/libkleidiai.so": No such file or
  directory.
Call Stack (most recent call first):
  build-kai/ggml/src/cmake_install.cmake:71 (include)
  build-kai/ggml/cmake_install.cmake:47 (include)
  build-kai/cmake_install.cmake:47 (include)

[chaxu01]

Using SOURCE_SUBDIR pathThatDoesNotExist to suppress target creation could be problematic. If CMake validates the path or the KleidiAI project layout changes, this could fail at configure time. An alternative would be to use FetchContent_Populate() instead of FetchContent_MakeAvailable(), and then not add the KleidiAI subdirectory at all.