package/swupdate: fix static build without lua

The lua_swupdate.so library was still built (without any object files)
and linked against swupdate even when HAVE_LUA was not set. This fails
in some static-only configurations.

Fixes:
 - http://autobuild.buildroot.org/results/c11c4d26983e0347d96f3dda62e6d72b031967bb



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>