Allow vk-generate.py to work in Python 2.

There were two places where this script called super, one with
Python 2 and 3 compatible syntax and one with Python 3 only. Fix
it to use the compatible syntax in both locations.
diff --git a/vk-generate.py b/vk-generate.py
index 6f478a6..860f994 100755
--- a/vk-generate.py
+++ b/vk-generate.py
@@ -192,7 +192,7 @@
         else:
             self.exports = library_exports[self.argv[1]]
 
-        super().run()
+        super(WinDefFileSubcommand, self).run()
 
     def generate_copyright(self):
         return """; THIS FILE IS GENERATED.  DO NOT EDIT.