check-kernel-headers: mktemp --tmpdir not available on RedHat RHEL5

mktemp --tmpdir is not available on older Redhat RHEL5 machines. The
alternative that has the same behavior is 'mktemp -t'.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>