Autogen: Don't change the order of HEADERS array in AutogenInfo.json
Qt6 reads our internal
AutogenInfo.json file. In !5624 (merged), 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 (closed)