netprio_cgroup: define sk_cgrp_prioidx only if NETPRIO_CGROUP is enabled

sock->sk_cgrp_prioidx won't be used at all if CONFIG_NETPRIO_CGROUP=n.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -367,7 +367,7 @@ struct sock {
 	unsigned short		sk_ack_backlog;
 	unsigned short		sk_max_ack_backlog;
 	__u32			sk_priority;
-#ifdef CONFIG_CGROUPS
+#if IS_ENABLED(CONFIG_NETPRIO_CGROUP)
 	__u32			sk_cgrp_prioidx;
 #endif
 	struct pid		*sk_peer_pid;