Fixes this:

    [3/4] Compiling C object 'src/miners/fs/8409082@@tracker-miner-fs@exe/tracker-writeback-dispatcher.c.o'.
    ../src/miners/fs/tracker-writeback-dispatcher.c: In function ‘writeback_dispatcher_writeback_file’:
    ../src/miners/fs/tracker-writeback-dispatcher.c:375:11: warning: assignment to ‘TrackerMinerFiles *’ {aka ‘struct TrackerMinerFiles *’} from incompatible pointer type ‘TrackerMinerFS *’ {aka ‘struct _TrackerMinerFS *’} [-Wincompatible-pointer-types]
      data->fs = g_object_ref (fs);