collectd: fix lua callback registration

collectd: fix lua callback registration

If you want to register more than one lua plugin, you cannot start the
collectd.

You get always the error message:

"The read function "lua/(null)" is already registered. Check for
duplicates in your configuration!"

This is not what I expect and what the documenation says.
With this change it is now possible to registrate more then one lua
Script.

See https://github.com/collectd/collectd/pull/2379

Signed-off-by: Florian Eckert <fe@dev.tdt.de>