package/linux: trash stderr in LINUX_VERSION_PROBED

The Linux buildsystem tries to run the compiler even just for
'kernelrelease' (which we store in LINUX_VERSION_PROBED) and we
sometimes need to use it before the toolchain is available; thus
we get spurious errors on stderr.

Consign stderr to oblivion when computing the 'kernelrelease'.

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