[CMAKE] Only build the bitcoin-qt application bundle on OSX

Summary:
Follow-up for the discussion in D4426.

When building for OSX, the bitcoin-qt target will build the application
bundle.

Test Plan:

ninja bitcoin-qt

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4437