[CMAKE] Add missing files to build bitcoin-qt on OSX

Summary:
The OSX specific files (*.mm) are missing in the OSX build.

Depends on D3032 (for the test plan).

Test Plan:

mkdir buildcmake && cd buildcmake
cmake -GNinja -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/OSX.cmake ..
ninja -v

Check the (*.mm) files are part of the build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3033