[NaCl SDK] Disable relative import python presubmit warnings.

The CQ fails otherwise.

BUG=none
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Review URL: https://codereview.chromium.org/813143002

Cr-Commit-Position: refs/heads/master@{#309102}
diff --git a/native_client_sdk/PRESUBMIT.py b/native_client_sdk/PRESUBMIT.py
index 4fd79cc..7e893f1 100644
--- a/native_client_sdk/PRESUBMIT.py
+++ b/native_client_sdk/PRESUBMIT.py
@@ -15,6 +15,7 @@
     'F0401',  # Unable to import module
     'R0401',  # Cyclic import
     'W0613',  # Unused argument
+    'W0403',  # relative import warnings
     'E1103',  # subprocess.communicate() generates these :(
     'R0201',  # method could be function (doesn't reference self)
   ]