[infra] Add missing import to linux_distribution_support.py

[email protected]

Change-Id: I00dfdc3257be8bd2ca9b39e957c427e842c98cd0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/170683
Reviewed-by: Alexander Thomas <[email protected]>

Author: athomas

tools/bots/linux_distribution_support.py

@@ -12,6 +12,7 @@
 
 import os
 import re
+import subprocess
 import sys
 
 import bot_utils