[CMAKE] Make Qt protobuf an object library

Summary:
That will allow for setting properties on these generated files that do
not apply to other Qt targets. This will be useful for preventing
clang-tidy from running on the generated files.

There is no change in behavior.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5121