[NET]: Fix "sysctl_net.c:36: error: 'core_table' undeclared here"

During the build for ARM machine type "fortunet", this error occurred:

  CC      net/sysctl_net.o
net/sysctl_net.c:36: error: 'core_table' undeclared here (not in a function)

It appears that the following configuration settings cause this error
due to a missing include:
CONFIG_SYSCTL=y
CONFIG_NET=y
# CONFIG_INET is not set

core_table appears to be declared in net/sock.h.  if CONFIG_INET were
defined, net/sock.h would have been included via:
  sysctl_net.c -> net/ip.h -> linux/ip.h -> net/sock.h

so include it directly.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>