Move #ifdef to avoid warnings when building with GStreamer <= 1.16

    ../src/tracker-writeback/tracker-writeback-gstreamer.c: In function ‘writeback_gstreamer_write_file_metadata’:
    ../src/tracker-writeback/tracker-writeback-gstreamer.c:954:32: warning: variable ‘algorithm’ set but not used [-Wunused-but-set-variable]
      954 |    const gchar *value = NULL, *algorithm;
          |                                ^~~~~~~~~
    ../src/tracker-writeback/tracker-writeback-gstreamer.c:954:17: warning: variable ‘value’ set but not used [-Wunused-but-set-variable]
      954 |    const gchar *value = NULL, *algorithm;
          |                 ^~~~~