tracker-extract: Fix critical warning in controller's finalize
tracker-extract: Fix critical warning in controller's finalize

I tested the ref count of priv->extractor in the finalize of the
controller, and it was 0 already at that point. Making me believe
that an ownership transfer is needed at set of the property.

outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker