gst: enable the pipewire ticks as a clock source

Use the pipewire ticks again as the clock source.

This was disabled because the v4l2 sources created bad ticks for the
graph. Now that this is improved we can enable the ticks again.

This has the advantage that simple audio playback does not drift
anymore. The only remaining problem would be timestamp drift or
discontinuities, which we don't handle here yet.

src/gst/gstpipewireclock.c

@@ -38,7 +38,7 @@ gst_pipewire_clock_get_internal_time (GstClock * clock)
     return pclock->last_time;
 
   now = pw_stream_get_nsec(s->pwstream);
-#if 0
+#if 1
   struct pw_time t;
   if (s->pwstream == NULL ||
       pw_stream_get_time_n (s->pwstream, &t, sizeof(t)) < 0 ||