cgroup.h: remove some functions that are now gone

cgroup_lock() and cgroup_unlock() are now no longer exported, so fix
cgroup.h to not declare them if CONFIG_CGROUPS is not enabled.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>