platform: ext: adi: define add_subdirectory_ifdef

remote_library.cmake uses FetchContent_MakeAvailable,
it auto include hal_adi, where the source CMakeLists
has add_subdirectory_ifdef that is not defined. Hence,
define it.

Change-Id: I12cc6bacb3d5523ddf09f3c11f2e94e16b490d92
Signed-off-by: Hao Zhang <Hao.Zhang@analog.com>
1 file changed