kselftest/arm64: Fix build warnings for ptrace

A "%s" is missing in ksft_exit_fail_msg(); instead, use the newly
introduced ksft_exit_fail_perror().

Signed-off-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240830052911.4040970-1-dev.jain@arm.com


Signed-off-by: Will Deacon <will@kernel.org>