Get executable to compile when gpl flag is disabled

I can sometimes get hlint to compile without this patch, but I'm not sure why. This seems to get things working reliably. An alternative patch which may be better is at:

https://github.com/fpco/hlint/commit/808b7b00f7cb30a7f68f94cc96a210c4f718d36b

I can update my pull request to use that if you prefer.