Fix a thing

Pretty sure this doesn't affect anything anymore, but I fixed it.

** (gnome-sudoku:23210): WARNING **: expected enumeration type
GtkWindowType, but got GtkMessageType instead
/home/mcatanzaro/jhbuild/install/lib/python3.3/site-packages/gnome_sudoku/gtk_goodies/dialog_extras.py:145:
Warning: value "((GtkWindowType) 2)" of type 'GtkWindowType' is invalid
or out of range for property 'type' of type 'GtkWindowType'
  GObject.GObject.__init__(self, *args, **kwargs)