Qt6 reads our internal AutogenInfo.json file. In 92d7b456
(Autogen: Add support for per-config sources, 2020-10-20), the
ordering of the HEADERS array elements was changed and this
broke their cmake_automoc_parser. We don't need to change the
order of the array elements, we can append to the existing ones
instead. Do this to allow their cmake_automoc_parser to continue
to work.

Fixes: #21823