No, you're not wasting your vote by voting Libertarian, unless the Party has stopped being libertarian
To the extent that an election is also an opinion poll, it's not really a wasted vote if you pick your true first choice. Let’s say the Libertarians get 7% of the vote. Whoever wins the election will see that and think, “Hmm, we have 7% of the electorate actually turning out to vote for Libertarians. I guess this is a group I’ll need to accommodate in some way if I want to govern and win more political support.”
The government does, after all, depend somewhat on the consent of the governed. If politicians are getting elected with small pluralities, e.g. in the 30s rather than the 50s, it starts to make them look bad and calls their legitimacy into question. Yes, they still command armies and police, but it becomes harder for them to make the moral case that they have a strong mandate. They need to make that argument in order to say, “What I do is not only possible, but right and even inevitable, given the will of the voters.”
Also, if enough people vote Libertarian, it becomes easier to argue that we need a proportional representation system, or that Libertarians should be allowed into the debates.
There is one situation that would make it a wasted vote, though, and that would be if the Libertarian Party stopped being libertarian. In that case, I would have to suggest writing in a candidate of your choice, even if it's yourself.