Grease Pencil: Fix using wrong flag for selecting objects on convert

The correct fix should be to use ED_object_base_select() but I'm leaving
this to a separate pass.

source/blender/editors/gpencil/gpencil_convert.c

@@ -1396,7 +1396,7 @@ static void gp_layer_to_curve(bContext *C,
   }
 
   /* set the layer and select */
-  base_new->flag |= SELECT;
+  base_new->flag |= BASE_SELECTED;
   BKE_scene_object_base_flag_sync_from_base(base_new);
 }