net: enetc: include ip6_checksum.h for csum_ipv6_magic

For those architectures which do not define_HAVE_ARCH_IPV6_CSUM, we need
to include ip6_checksum.h which provides the csum_ipv6_magic() function.

Fixes: fb8629e2 ("net: enetc: add support for software TSO")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20211012121358.16641-1-ioana.ciornei@nxp.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>