plug-ins: fix usage of gimp_file_save() in py-slice.py.

Signature changed (commit f3fb3d1a), but build could not catch the
missing usage in non-compiled scripts. Thanks to Massimo for noticing
it.

plug-ins/python/py-slice.py

@@ -196,7 +196,8 @@ def slice(image, drawable, image_path, image_basename, image_extension,
     if image_extension == "jpg" and image.base_type() == Gimp.ImageBaseType.INDEXED:
         temp_image.convert_rgb ()
 
-    Gimp.file_save(Gimp.RunMode.NONINTERACTIVE, temp_image, temp_drawable, filename, filename)
+    Gimp.file_save(Gimp.RunMode.NONINTERACTIVE, temp_image, temp_drawable,
+                   Gio.file_new_for_path (filename))
 
     temp_image.delete()
     return src