Make t-dn also work for Qt 5
* src/CMakeLists.txt (qgpgme_PRIVATE_HEADERS): Add qt6compat_p.h for Qt 5 builds. * src/dn.h: Explicitly declare metatype for DN::Attribute. * src/qt6compat_p.h: New. Copied from kleopatra (gpg4win/24.05). * tests/CMakeLists.txt: Build t-dn test also for Qt 5. * tests/t-dn.cpp (operator==): Put the operator in the QGpgME namespace to fix Qt 5 build.