fix build with uClibc without IPv6

with [buildroot](http://buildroot.net/), we use various C libraries, for example [uClibc](http://www.uclibc.org/) which could be compiled with or without IPv6.

this commit solves this build issue : http://autobuild.buildroot.net/results/698/6989e71bf50946d62b696dbe07e8f00679b6aad9/build-end.log