profiler: Set 'persistent' state when persistent

This makes sure calling Start() and Stop() doesn't work, since we're
always recording.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2846>

src/core/meta-profiler.c

@@ -219,6 +219,7 @@ meta_profiler_init (MetaProfiler *self)
       cogl_set_tracing_enabled_on_thread (main_context,
                                           group_name,
                                           env_trace_file);
+      self->persistent = TRUE;
     }
 }