On cygwin:

  CC        libvirt_util_la-cgroup.lo
util/cgroup.c: In function 'virCgroupKillRecursiveInternal':
util/cgroup.c:1458: warning: implicit declaration of function 'virCgroupNew' [-Wimplicit-function-declaration]

* src/util/cgroup.c (virCgroupKill): Don't build on platforms
where virCgroupNew is unsupported.