PlaylistDAO: Add missing spaces in SQL query

[uklotzde]

Fixup of #2900, didn't notice in the logs:

Warning [Main]: /tmp/mixxx/src/library/dao/playlistdao.cpp 413 FAILED QUERY [ "SELECT p1.position FROM PlaylistTracks AS p1WHERE p1.id NOT IN (SELECT p2.id FROM PlaylistTracks AS p2INNER JOIN library ON library.id = p2.track_id WHERE p2.playlist_id = p1.playlist_id AND library.mixxx_deleted = 0 ) AND p1.playlist_id=:id" ] QSqlError("", "Parameter count mismatch", "")

That's the reason why I prefer to use VERIFY_OR_DEBUG_ASSERT around query preparation and execution to detect such errors early.

[Holzhaus]

LGTM, waiting for CI.