Install headers to path prefix matching C++ namespace Kirigami
See discussion https://marc.info/?l=kde-frameworks-devel&m=164132162218815&w=2 ff.
This set of patches fixes things, based on similar work done around improving/fixing include paths for KDNSSD and Akonadi.
Currently the deprecation warnings are not emitted though, as Kirigami introduced ecm_generate_export_header without specifying the group base names "KF" and giving a DEPRECATED_BASE_VERSION, so deprecation warnings default to be emitted only starting for current version (5.90 still as not bumped) and ignore the 'KF_*' control variables. That ideally should be fixed (in a backward compatible way), but left for others.