Fix build with gcc 4.8

When building master with gcc 4.8, the build fails with errors like
this:
error: no matching function for call to
'MediaPlayer2Player::connect(QPointer<PlayerManager>&, void
(PlayerManager::*)(const FileHandle&), MediaPlayer2Player* const, void
(MediaPlayer2Player::*)()const)'

See also https://bugreports.qt.io/browse/QTBUG-48988

This patch fixes it.

The fix is similar to fixes in other applications, e.g.
https://cgit.kde.org/kdevelop.git/commit/?h=5.2&id=6d5bdbba0a04d7180bda1f6bb47acb386575a561

Differential Revision: https://phabricator.kde.org/D8716