[IPV4]: Remove bogus ifdef mess in arp_process

The #ifdef's in arp_process() were not only a mess, they were also wrong 
in the CONFIG_NET_ETHERNET=n and (CONFIG_NETDEV_1000=y or 
CONFIG_NETDEV_10000=y) cases.

Since they are not required this patch removes them.

Also removed are some #ifdef's around #include's that caused compile 
errors after this change.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>