Minor fix to configure on s390x

configure on s390x has a key check fail with an error about
a variable being used uninitialized. So let's initialize it.

Reviewed-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12126