commit ffd756b3 upstream.

Unlike for IPv6, the IPv4 checksum functions are only available
if CONFIG_INET is set.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>