Reapply 0d9208ce on 5.11

This change amends 305dd1b6, which lost
0d9208ce and brought src/plugins/styles/mac/qmacstyle_mac.mm back. In
4f3249f3

, it was moved to
src/plugins/styles/mac in 5.10.

Task-number: QTBUG-65773
Change-Id: I721268caf12067ed798f5846234cd2fdf3e493dc
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>
Reviewed-by: Jake Petroules <jake.petroules@qt.io>