libuci: fix Lua finding

This patch fixes Lua path comparison, if Lua is not provided.

Fixes

  http://autobuild.buildroot.org/results/a14/a140e9ea33202e8b870ec047a573f269e9a74b08/



[Thomas: add reference to the autobuilder failure.]

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>