Test build failure for x86_64-unknown-fuchsia

[boris-n]

Running "make test_all" on master currently fails:

error[E0599]: no method named original_dst_v6 found for struct Socket in the current scope
--> tests/socket.rs:1738:18
|
1738 | match socket.original_dst_v6() {
| ^^^^^^^^^^^^^^^
|

The reason seems to be a wrong cfg in the test: original_dst_v6 is defined in src/socket.rs for android, linux and windows but the test in tests/socket.rs checks for android, fuchsia and linux.

[boris-n]

Please review and merge, so "make test_all" works again.