Fix the following W=1 kernel build warning(s):

 net/netlabel/netlabel_mgmt.c:78: warning: expecting prototype for netlbl_mgmt_add(). Prototype was for netlbl_mgmt_add_common() instead
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>