Portable Python script across Python version

Using from __future__ import print_function it is possible to have a compatible behavior of `print(...)` across Python version.

Differential Revision: https://reviews.llvm.org/D55213

git-svn-id: svn://svn.chromium.org/llvm-project/cfe/trunk/tools/clang-format@349454 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
diff --git a/clang-format-diff.py b/clang-format-diff.py
index ce4c1d6..1721d8a 100755
--- a/clang-format-diff.py
+++ b/clang-format-diff.py
@@ -21,6 +21,7 @@
   svn diff --diff-cmd=diff -x-U0 | clang-format-diff.py -i
 
 """
+from __future__ import print_function
 
 import argparse
 import difflib