Python 3 removes the reduce builtin

This fix is merely the result of the suggestion in the Python 3 docs.
See: https://docs.python.org/3.0/whatsnew/3.0.html#builtins

Author: RLovelett

utils/update-checkout

@@ -16,6 +16,8 @@ import json
 import os
 import sys
 
+from functools import reduce
+
 sys.path.append(os.path.dirname(__file__))
 
 from SwiftBuildSupport import (