USD Exporter: show export options by default

There are options for the exporter that are relevant for each export, for
example the choice between Viewport or Render settings, or whether to
export the current frame or an animation. It's better to have the options
panel opened by default.

source/blender/editors/io/io_usd.c

@@ -176,7 +176,7 @@ void WM_OT_usd_export(struct wmOperatorType *ot)
                                  FILE_TYPE_FOLDER | FILE_TYPE_USD,
                                  FILE_BLENDER,
                                  FILE_SAVE,
-                                 WM_FILESEL_FILEPATH,
+                                 WM_FILESEL_FILEPATH | WM_FILESEL_SHOW_PROPS,
                                  FILE_DEFAULTDISPLAY,
                                  FILE_SORT_ALPHA);