ipv4: register igmp_notifier even when !CONFIG_PROC_FS

We still need this notifier even when we don't config
PROC_FS.

It should be rare to have a kernel without PROC_FS,
so just for completeness.

Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Patrick McHardy <kaber@trash.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/ip.h
Simple merge

diff --cc net/ipv4/igmp.c
Simple merge

diff --cc net/ipv4/ip_output.c
Simple merge