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: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>