Fix compile error
Fix compile error

```
╭─mindcat@mindcat-linux-pc /tmp/mio  ‹master› 
╰─➤  cargo build                                                                                                                                                 101 ↵
   Compiling mio v0.2.1 (file:///tmp/mio)
src/os/posix.rs:240:17: 240:36 error: unresolved name `nix::SockAddr::IpV4`
src/os/posix.rs:240     let mut a = nix::SockAddr::IpV4(sa);
                                    ^~~~~~~~~~~~~~~~~~~
src/os/posix.rs:249:17: 249:36 error: unresolved name `nix::SockAddr::IpV4`
src/os/posix.rs:249     let mut a = nix::SockAddr::IpV4(sa);
                                    ^~~~~~~~~~~~~~~~~~~
src/os/posix.rs:287:9: 287:28 error: unresolved enum variant, struct or const `IpV4`
src/os/posix.rs:287         nix::SockAddr::IpV4(sin) => {
                            ^~~~~~~~~~~~~~~~~~~
src/os/posix.rs:290:9: 290:28 error: unresolved enum variant, struct or const `Unix`
src/os/posix.rs:290         nix::SockAddr::Unix(addr) => {
                            ^~~~~~~~~~~~~~~~~~~
src/os/posix.rs:316:21: 316:40 error: unresolved name `nix::SockAddr::IpV4`
src/os/posix.rs:316                     nix::SockAddr::IpV4(addr)
                                        ^~~~~~~~~~~~~~~~~~~
src/os/posix.rs:332:13: 332:32 error: unresolved name `nix::SockAddr::Unix`
src/os/posix.rs:332             nix::SockAddr::Unix(addr)
                                ^~~~~~~~~~~~~~~~~~~
error: aborting due to 6 previous errors
Could not compile `mio`.

To learn more, run the command again with --verbose.
```
fhartwig/mio,greensnark/mio,geofft/mio,jamwt/mio,carllerche/mio,tailhook/mio,yuikns/mio,thijsc/mio,dwrensha/mio,carllerche/mio,luca-barbieri/mio,murarth/mio,bottompawn/mio,rryan/mio,zslayton/mio,echochamber/mio,yangxingpping/mio,dpc/mio,rrichardson/mio,mneumann/mio,hjr3/mio,fuchsia-mirror/third_party-mio,pythonesque/mio,alexcrichton/mio,fuchsia-mirror/third_party-mio,asomers/mio,marjakm/mio,tempbottle/mio,frewsxcv/mio,mlalic/mio,danburkert/mio,arthurprs/mio,jamwt/mio,asomers/mio,mneumann/mio