cgroup: Fix build warnings when CGROUP not defined

cgroup: Fix build warnings when CGROUP not defined

Commit 57114e95e8 ("cgroup: refactor allow_attach function into common
code") unnecessarily added a dummy definition for
subsys_cgroup_allow_attach when CONFIG_CGROUP is not set, so remove it
to prevent warnings like...

include/linux/cgroup.h:907:18: warning: 'struct cgroup_taskset' declared inside parameter list
           struct cgroup_taskset *tset)
                  ^
include/linux/cgroup.h:907:18: warning: its scope is only this definition or declaration, which is probably not what you want
include/linux/cgroup.h:907:18: warning: 'struct cgroup' declared inside parameter list
  CC      kernel/events/core.o
In file included from kernel/events/core.c:41:0:
include/linux/cgroup.h:907:18: warning: 'struct cgroup_taskset' declared inside parameter list
           struct cgroup_taskset *tset)

Signed-off-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>