This is a tricky question, but luckily...
Based on other Metas, I don't think the answer to this question matters
The answer to this question would be the basis for acting on this meta:
Should we be downvoting conventional answers?
The answer there was pretty much "no", though perhaps there is some more discussion to be had there. But going with that answer, it seems that it doesn't really matter if a solution is hacky or not... you vote for it based on its merits as a useful, efficient solution.
This Meta might also be relevant:
Do questions asking for common-sense advice really count as asking about lifehacks?
The answer is "Yes", mainly because what is common-sense for one person might not be for another, which again shows that the answer to "What is a hack?" doesn't seem to matter.
Leave the definition subjective, since it at most affects your upvoting/downvoting, which is already subjective
I think those two questions kind of point towards the answer to your question not mattering. Draw the line for yourself whether you think something is really a lifehack. This will be a subjective process just as the entire upvoting/downvoting process is, and since it sounds like about the only effect it will have is how you upvote/downvote (I mean, we can tell people not to downvote for certain things in Meta, but that's not going to stop everyone), I think it's perfectly reasonable to leave the distinction of "Is this hacky or not?" subjective and not try to lay out rules for what counts as a hack. Such rules would only matter if we were closing questions or flagging answers based on it. Unless such a policy gets traction, we don't really need to try to define whether or not something is truly a lifehack.