codegen: Explicitly close output

This is just cleaner rather than relying on the GC, and maybe
if we're lucky it will actually solve a problem.

https://bugzilla.gnome.org/show_bug.cgi?id=684909

gio/gdbus-2.0/codegen/codegen_main.py

@@ -194,6 +194,8 @@ def codegen_main():
                                     docbook_gen,
                                     h, c);
         ret = gen.generate()
+        h.close()
+        c.close()
 
     sys.exit(0)