cmake : check if KleidiAI API has been fetched (llama/19640)

cmake : check if KleidiAI API has been fetched (llama/19640)

This commit addresses a build issue with the KleidiAI backend when
building multiple cpu backends. Commmit
3a00c98584e42a20675b6569d81beadb282b0952 ("cmake : fix KleidiAI install
target failure with EXCLUDE_FROM_ALL") introduced a change where
FetchContent_Populate is called instead of FetchContent_MakeAvailable,
where the latter does handle this case (it is idempotent but
FetchContent_Populate is not).

I missed this during my review and I should not have commited without
verifying the CI failure, sorry about that.