ggml-cpu : prevent kleidiai build/install targets

[anyj0527]

• currently cmake install targets of kleidiai generate error
• use FetchContent_Populate to disable all build/install targets of kleidiai
• note that cmake would complains about the deprecation of FetchContent_Populate

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.

[anyj0527]

@chaxu01 Thanks for your clean review.

At first I consider FetchContent_Populate which do the same fix but cmake generates deprecation warning about it. Then I found the SOURCE_SUBDIR bypass.

Still your concern is valid, I've updated this commit to use FetchContent_Populate, PTAL