Bug 1543663 - Add missing 'import errno' to fix some python-tests; r=ahal

Bug 1537574 moved some code that uses errno from building.py to base.py,
but the latter didn't already import errno.

Differential Revision: https://phabricator.services.mozilla.com/D27125

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozbuild/base.py

@@ -12,6 +12,7 @@ import os
 import subprocess
 import sys
 import types
+import errno
 try:
     from shutil import which
 except ImportError: