pktgen: Require CONFIG_INET due to use of IPv4 checksum function

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>