Use relative path to fetch KleidiAI dependencies

The `CMAKE_SOURCE_DIR` variable always corresponds to the top level directory of the CMakefile being processed by CMake. This causes issues for CMake projects that fetch KleidiAI using `FetchContent()` as it incorrectly assumes KleidiAI's dependencies reside in that project's top level directory, rather than in KleidiAI's source tree. Resolve this issue by using the `CMAKE_CURRENT_SOURCE_DIR` variable to use relative paths to the KleidiAI project.

Signed-off-by: Jakub Sujak <jakub.sujak@arm.com>

Approved-by: Felix Johnny Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>