net: netprio_cgroup: make net_prio_subsys static

net_prio_subsys can be made static this removes the sparse
warning it was throwing.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>