include: Silence external kernel version checks

During the initial configuration phases, we have not set-up the kernel
source directory, which would lead to such messages:

cat:
/local/users/fainelli/openwrt/trunk/build_dir/target-x86_64_musl/linux-uml/linux-4.9.58/include/config/kernel.release:
No such file or directory

Just silence it, since it does not create a functional problem.

Fixes: 8e0e0e7d ("include: Determine MODULES_DIR correctly for external/git kernels")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>