scons: Add os import to marshall

This file was missing the `import os` after
I3ad28b6ee52fd347d2fe71f279baab629e88d12c

Change-Id: I7fde59e92f03fd240f48a304488d77628bfdb852
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/70918
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
diff --git a/build_tools/marshal.py b/build_tools/marshal.py
index 4a1522f..58c78e1 100644
--- a/build_tools/marshal.py
+++ b/build_tools/marshal.py
@@ -50,6 +50,7 @@
 
 import locale
 import marshal
+import os
 import sys
 import zlib