net: make symbol 'flush_works' static

The sparse tool complains as follows:

net/core/dev.c:5594:1: warning:
 symbol '__pcpu_scope_flush_works' was not declared. Should it be static?

'flush_works' is not used outside of dev.c, so marks
it static.

Fixes: 41852497 ("net: batch calls to flush_all_backlogs()")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -5591,7 +5591,7 @@ void netif_receive_skb_list(struct list_head *head)
 }
 EXPORT_SYMBOL(netif_receive_skb_list);
 
-DEFINE_PER_CPU(struct work_struct, flush_works);
+static DEFINE_PER_CPU(struct work_struct, flush_works);
 
 /* Network device is going away, flush any packets still pending */
 static void flush_backlog(struct work_struct *work)