ipconfig: move ic_dev_xid under IPCONFIG_BOOTP

ic_dev_xid is only used in __init ic_bootp_recv under IPCONFIG_BOOTP
and __init ic_dynamic under IPCONFIG_DYNAMIC(which is itself defined
with the same IPCONFIG_BOOTP)

This patch fixes the following warning when IPCONFIG_BOOTP is not set:
>> net/ipv4/ipconfig.c:146:15: warning: 'ic_dev_xid' defined but not used [-Wunused-variable]
    static __be32 ic_dev_xid;  /* Device under configuration */

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: netdev@vger.kernel.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David S. Miller <davem@davemloft.net>