[test] Include IPv6 support when performing settings self-tests

The settings self-tests include tests for the "ipv6" setting type.
When IPv6 support is not included, this setting type exists (since it
is referred to by some dual-stack code, such as dns.c) but is
non-functional.

Force IPv6 support to be included within a settings self-test build
using an explicit REQUIRE_OBJECT() macro.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/tests/settings_test.c

 	.name = "settings",
 	.exec = settings_test_exec,
 };
+
+/* Include real IPv6 setting type */
+REQUIRING_SYMBOL ( settings_test );
+REQUIRE_OBJECT ( ipv6 );