libtracker-common: Fix building Tracker as a subproject

The module lookup is currently based to the top builddir, which
might differ is Tracker is being built as a subproject. Make
it dependent on the current build dir (i.e. the directory the
module will be built to), so it works also on those situations.

Fixes: a4f9885a ("libtracker-common: Convert unicode parser to a GModule")