kselftest/arm64: Fix ABI header directory location

Currently the arm64 kselftests attempt to locate the ABI headers using
custom logic which doesn't work correctly in the case of out of tree builds
if KBUILD_OUTPUT is not specified. Since lib.mk defines KHDR_INCLUDES with
the appropriate flags we can simply remove the custom logic and use that
instead.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220503231655.211346-1-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>