support/check-kernel-headers: use mktemp instead of hand-crafted temp file

Instead of creating a temporary files with a dubious scheme, use mktemp,
which purpose is exactly that: creating temporary files

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>