lighttpd: fix lua compatibility with 5.2.x / 5.3.x

By backporting a fix from upstream.

Fixes:
http://autobuild.buildroot.net/results/13f/13faa9ebce00359b8943775364a432ce422fe8b8/



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