libtracker-extract: define back extract[rules|modules]dir for Makefile.am to use

Before commit 73c0decd, src/libtracker-extract/Makefile.am was receiving those
indirectly from $(topsrcdir)/Makefile.decls. After that file was no longer included
those remained empty, causing wrong file lookups in the built
tracker-miner-fs and tracker-extract.

So the declarations have been moved to a separate $(topsrcdir)/Makefile.extractdirs,
so they're defined once and available to both src/libtracker-extract/Makefile.am and
src/tracker-extract/Makefile.am.
libtracker-miner: Protect against NULL thumbnailer

If the service initialization fails, a NULL object will be returned,
so check for that before attempting to communicate with the thumbnailer.