scons: Use print_function ins SConstruct

This ensures that we get python3's print() function behavior even in
python2, instead of python2's print statement behavior. We'll be using
this in the next patch.

Reviewed-by: Eric Anholt <eric@anholt.net>

SConstruct

@@ -20,6 +20,7 @@
 # to get the full list of options. See scons manpage for more info.
 #
 
+from __future__ import print_function
 import os
 import os.path
 import sys