luasocket: fix build with uClibc without IPv6

see http://autobuild.buildroot.net/results/698/6989e71bf50946d62b696dbe07e8f00679b6aad9/

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>