Fix commit 7cc88aff2eecfe875951cc689824c96ae8e0e2d1 (check if add-on is actually enabled)

See [here](https://github.com/XBMC-Addons/service.xbmc.versioncheck/commit/7cc88aff2eecfe875951cc689824c96ae8e0e2d1#diff-a534c3c8963e6e7695f1d2cd016b98c7L37) for details.