netfilter: ipt_CLUSTERIP: fix build error without procfs

We can't access c->pde if CONFIG_PROC_FS is disabled:

net/ipv4/netfilter/ipt_CLUSTERIP.c: In function 'clusterip_config_find_get':
net/ipv4/netfilter/ipt_CLUSTERIP.c:147:9: error: 'struct clusterip_config' has no member named 'pde'

This moves the check inside of another #ifdef.

Fixes: 6c5d5cfbe3c5 ("netfilter: ipt_CLUSTERIP: check duplicate config when initializing")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>