OBJ: Enable undo for experimental OBJ importer

The new OBJ importer operator didn't register an undo event.
This commit enables the register and undo flags for the operator.

Reviewed By: Bastien Montagne, Aras Pranckevicius, Serhiy Striletksy
Differential Revision: https://developer.blender.org/D15051

source/blender/editors/io/io_obj.c

@@ -456,6 +456,7 @@ void WM_OT_obj_import(struct wmOperatorType *ot)
   ot->name = "Import Wavefront OBJ";
   ot->description = "Load a Wavefront OBJ scene";
   ot->idname = "WM_OT_obj_import";
+  ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
 
   ot->invoke = wm_obj_import_invoke;
   ot->exec = wm_obj_import_exec;