arp: fix unused variable warnning when CONFIG_PROC_FS=n

net/ipv4/arp.c:1412:36: warning: unused variable 'arp_seq_ops' [-Wunused-const-variable]

Add #ifdef CONFIG_PROC_FS for 'arp_seq_ops'.

Fixes: e968b1b3 ("arp: Remove #ifdef CONFIG_PROC_FS")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>