Build API: Update imports to new generated proto

BUG=chromium:940582, b:128456793
TEST=run_tests

Change-Id: I34645640bf4fa07b4403f9e2e5e596b7a59dff97
Reviewed-on: https://chromium-review.googlesource.com/1525076
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: Lann Martin <lannm@chromium.org>
diff --git a/api/controller/image.py b/api/controller/image.py
index 046b875..c734d99 100644
--- a/api/controller/image.py
+++ b/api/controller/image.py
@@ -12,7 +12,7 @@
 
 import os
 
-from chromite.api.gen import image_pb2
+from chromite.api.gen.chromite.api import image_pb2
 from chromite.lib import constants
 from chromite.lib import image_lib
 from chromite.service import image