build: fix cgroups on non-Linux
    
    Running ./autobuild.sh detected a mingw failure:
    
      CCLD     libvirt.la
    Cannot export virCgroupGetPercpuStats: symbol not defined
    Cannot export virCgroupSetOwner: symbol not defined
    
    * src/util/vircgroup.c (virCgroupGetPercpuStats)
    (virCgroupSetOwner): Implement stubs.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>