tools: fix another deprecation warning, this time in PyGObject.

Fixes:

> PyGTKDeprecationWarning: Using positional arguments with the GObject constructor has been deprecated.
> Please specify keyword(s) for "label" or use a class specific constructor.
> See: https://wiki.gnome.org/PyGObject/InitializerDeprecations

tools/performance-log-viewer.py

@@ -1557,7 +1557,7 @@ class SampleGraphList (Gtk.Box):
                 label.show ()
 
                 if marker.description:
-                    label = Gtk.Label (marker.description,
+                    label = Gtk.Label (label  = marker.description,
                                        halign = Gtk.Align.END)
                     grid.attach (label, 1, row, 1, 1)
                     label.show ()