build: Fix configure error in tracker_core=subproject mode

Fixes the following error from Meson:

../meson.build:459:21: ERROR: Unknown variable "tracker_sparql_uninstalled_dir".

Nothing changed recently here but it looks like we didn't correctly query that variable from the 'tracker' subproject, relying on it being implicitly available. With Meson 0.55.3 this wasn't working, but perhaps it worked with earlier versions.