net: ipv4: ipconfig: fix unused variable

When CONFIG_PROC_FS isn't set, variable ipconfig_dir isn't used.
net/ipv4/ipconfig.c:167:31: warning: ‘ipconfig_dir’ defined but not used [-Wunused-variable]
 static struct proc_dir_entry *ipconfig_dir;
                               ^~~~~~~~~~~~
Move the declaration of ipconfig_dir inside the CONFIG_PROC_FS ifdef to
fix the warning.

Fixes: c04d2cb2

 ("ipconfig: Write NTP server IPs to /proc/net/ipconfig/ntp_servers")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>