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

lib/checksum_kunit.c

@@ -593,6 +593,7 @@ static void test_ip_fast_csum(struct kunit *test)
 
 static void test_csum_ipv6_magic(struct kunit *test)
 {
+#if defined(CONFIG_NET)
 	const struct in6_addr *saddr;
 	const struct in6_addr *daddr;
 	unsigned int len;
@@ -616,6 +617,7 @@ static void test_csum_ipv6_magic(struct kunit *test)
 		CHECK_EQ(expected_csum_ipv6_magic[i],
 			 csum_ipv6_magic(saddr, daddr, len, proto, csum));
 	}
+#endif /* !CONFIG_NET */
 }
 
 static struct kunit_case __refdata checksum_test_cases[] = {