Reviewers know very well that their suggestions and comments may end up in the final publication. This is the point of reviewing, and no reviewer would (or should) consider it plagiarism if sentences are copied verbatim. Having said that, it is always best to err on the side of caution, so the best course of action would be to let the editor know exactly what you did. You will contact the editor anyway with the revisions, and it is only natural to state that you used the reviewer's quote.
If you want, you can acknowledge the "anonymous reviewer" or ask the editor, and thereby indirectly the reviewer, whether they wish to be acknowledged by name (they will probably decline, but they would be happy at being offered the choice).
Personally, I once was acknowledged as "anonymous reviewer" for a model proposed based on the data in the paper (they used an entire figure I sent them). I fully expected the authors to use my image and model (why else would I have sent it?), but the acknowledgement came as a surprise and made me happy.