src/lib/env.c: Fix compilation on RHEL5

This is a fix to compile vzctl on RHEL5 after the previous
commit c1e40b0. Unfortunately, RHEL5's kernel-headers (or any
other package) does not provide linux/magic.h. Since it is just
one define, let's add it explicitly.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>