lib: checksum: Fix build with CONFIG_NET=n

The generic ipv6 checksums are only defined with CONFIG_NET=y, so gate
the test as well.

Fixes: 6f4c45cb ("kunit: Add tests for csum_ipv6_magic and ip_fast_csum")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401192143.jLdjbIy3-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202401192357.WU4nPRdN-lkp@intel.com/


Reviewed-By: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20240119145600.3093-2-palmer@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>