ToolInvoker._get_alignments: fix missing self parameter

Caught by pylint. I wonder if we need this piece of code as nobody every
triggered it.

Reviewed-by: James Cameron <quozl@laptop.org>
Acked-by: Simon Schampijer <simon@laptop.org>
CC: Aleksey Lim <alsroot@member.fsf.org>

src/sugar/graphics/palettewindow.py

@@ -853,7 +853,7 @@ class ToolInvoker(WidgetInvoker):
     def _get_alignments(self):
         parent = self._widget.get_parent()
         if parent is None:
-            return WidgetInvoker._get_alignments()
+            return WidgetInvoker._get_alignments(self)
 
         if parent.get_orientation() is gtk.ORIENTATION_HORIZONTAL:
             return self.BOTTOM + self.TOP