common/util: enable cgroup only on linux

common/util: enable cgroup only on linux

cgroups, i.e. control groups, are a linux kernel feature. this is not
available on FreeBSD, Win32 or MacOS. furthermore, we are reading the
information of it by accessing files exposed using a tmpfs filesystem
mounted at /sys/fs/cgroup, which is not available on other systems.

in this change, instead of guarding it using `#ifndef _WIN32`, it is now
guraded using `defined(__linux__)`.

Signed-off-by: Kefu Chai <kchai@redhat.com>