reorder includes to compile on OpenBSD

From getifaddr(3) manual:
```
If both <net/if.h> and <ifaddrs.h> are being
included, <net/if.h> must be included before <ifaddrs.h>
```

http://www.openbsd.org/cgi-bin/man.cgi?query=getifaddrs&apropos=0&sektion=0&manpath=OpenBSD+Current&arch=i386&format=html