Use Q_EMIT instead of emit

In C++20 there are new classes that have member functions named emit().
c.f.:
https://lists.qt-project.org/pipermail/development/2020-February/038812.html
https://en.cppreference.com/w/cpp/io/basic_osyncstream/emit

A similar change was done in the Frameworks:
https://invent.kde.org/frameworks/kio/-/merge_requests/315

src/BookmarkHandler.cpp

@@ -60,12 +60,12 @@ BookmarkHandler::~BookmarkHandler() = default;
 
 void BookmarkHandler::openBookmark(const KBookmark &bm, Qt::MouseButtons, Qt::KeyboardModifiers)
 {
-    emit openUrl(bm.url());
+    Q_EMIT openUrl(bm.url());
 }
 
 void BookmarkHandler::openFolderinTabs(const KBookmarkGroup &group)
 {
-    emit openUrls(group.groupUrlList());
+    Q_EMIT openUrls(group.groupUrlList());
 }
 
 bool BookmarkHandler::enableOption(BookmarkOption option) const