Fix T94366: Grease Pencil Automerge no immediate UI update

Just an oversight in rBe9607f45d85d.
Now add notifier that toolsettings changed.

Maniphest Tasks: T94366

Differential Revision: https://developer.blender.org/D13723

source/blender/makesrna/intern/rna_scene.c

@@ -3262,6 +3262,7 @@ static void rna_def_tool_settings(BlenderRNA *brna)
       "Automerge",
       "Join by distance last drawn stroke with previous strokes in the active layer");
   RNA_def_property_clear_flag(prop, PROP_ANIMATABLE);
+  RNA_def_property_update(prop, NC_SCENE | ND_TOOLSETTINGS, NULL);
 
   prop = RNA_def_property(srna, "gpencil_sculpt", PROP_POINTER, PROP_NONE);
   RNA_def_property_pointer_sdna(prop, NULL, "gp_sculpt");