[NET]: Fix GCC4 compile error: sysctl in linux/if_ether.h

[NET]: Fix GCC4 compile error: sysctl in linux/if_ether.h

The following is generated when compiling a
recent (2.6.14-rc2-git5) kernel configured for
ARM, with GCC4.

  CC      init/main.o
In file included from include/linux/netdevice.h:29,
                 from include/net/sock.h:48,
                 from init/main.c:50:
include/linux/if_ether.h:114: error: array type has incomplete element type

It seems that if CONFIG_SYSCTL is not set, then
the compiler will throw an error due to the definition
of the ether_table[] array

Attached is a solution to the problem

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: David S. Miller <davem@davemloft.net>