Fix typo in Qt5CoreMacros.cmake

According to the documentation, the argument is called COPYONLY instead
of COPY_ONLY.
Fixes warning and ensures it works properly.

Change-Id: I643f5ea808aaaf94c3ee666ec39485e84ed38df1
Reviewed-by: Vishesh Handa <vhanda@kde.org>
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>