scons: Import "sys" which is used in an exception handler.

If the call to "raw_input" fails (it does in python 3.x), then the
"except" runs and will also fail because sys hasn't been imported.

Change-Id: Ibf5778a893a5bd8aad17f4aee544ddcfe5085cab
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27947
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

site_scons/site_tools/git.py

@@ -40,6 +40,7 @@
 
 from __future__ import print_function
 import os
+import sys
 
 import gem5_scons.util
 from m5.util import readCommand