net: igmp: make function __ip_mc_inc_group() static

Fixes the following sparse warnings:

net/ipv4/igmp.c:1391:6: warning:
 symbol '__ip_mc_inc_group' was not declared. Should it be static?

Fixes: 6e2059b5 ("ipv4/igmp: init group mode as INCLUDE when join source group")
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -1387,7 +1387,8 @@ static void ip_mc_hash_remove(struct in_device *in_dev,
 /*
  *	A socket has joined a multicast group on device dev.
  */
-void __ip_mc_inc_group(struct in_device *in_dev, __be32 addr, unsigned int mode)
+static void __ip_mc_inc_group(struct in_device *in_dev, __be32 addr,
+			      unsigned int mode)
 {
 	struct ip_mc_list *im;
 #ifdef CONFIG_IP_MULTICAST