Correct /dev/null permissions on older kernels

Older kernels rely on udev to set proper permissions on /dev/null.
Commit 378f2e074e38490103f999173bf717a5394170f8 by Kay Sievers removed
the rule that set these permissions, which caused a regression. We
reintroduce that rule to restore compatibility.

Signed-off-by: Richard Yao <ryao@gentoo.org>