[CMAKE] Silent the Qt translation files generation

Summary:
This makes the build output much cleaner. Unfortunately, this is only
possible with Qt >= 5.11.
https://github.com/qt/qttools/commit/9f3dba2ea3978936565aa5e7893415a8a765268e

Test Plan:
With Qt >= 5.11:

ninja

Check the Generating bitcoin_xx_XX.qm lines are no longer displayed
(unless there is an additional info to display, such as `Removed plural
forms as the target language has less forms.`).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4682