cgroup: net_prio: Do not define task_netpioidx() when not selected

task_netprioidx() should not be defined in case the configuration is
CONFIG_NETPRIO_CGROUP=n. The reason is that in a following patch the
net_prio_subsys_id will only be defined if CONFIG_NETPRIO_CGROUP!=n.
When net_prio is not built at all any callee should only get an empty
task_netprioidx() without any references to net_prio_subsys_id.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Cc: Gao feng <gaofeng@cn.fujitsu.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Cc: netdev@vger.kernel.org
Cc: cgroups@vger.kernel.org