[qt6] Fix compilation of notification monitor

[jmlich]

/home/jmlich/workspace/ubports/libwatchfish/notificationmonitor.cpp: In member function ‘watchfish::ProtoNotification watchfish::NotificationMonitorPrivate::parseNotifyCall(DBusMessage*) const’:
/home/jmlich/workspace/ubports/libwatchfish/notificationmonitor.cpp:281:29: error: ‘class QHash<QString, QString>’ has no member named ‘unite’
  281 |                 proto.hints.unite(hints);
      |                             ^~~~~
gmake[2]: *** [CMakeFiles/libwatchfish.dir/build.make:79: CMakeFiles/libwatchfish.dir/notificationmonitor.cpp.o] Error 1

[jmlich]

qt6 fix included to other pull request