Fix unittest build.

The recent change to return the last error code broke the unittest
build.

Bug: 25598547
TEST=FEATURES=test emerge-link update_engine

Change-Id: I8fce1ded54a7a941cb3ac1d6a150217a8994bf5a
diff --git a/update_engine.gyp b/update_engine.gyp
index 7da34fb..b7aaa9d 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -333,8 +333,8 @@
         'libupdate_engine_client',
       ],
       'sources': [
-        'update_engine_client.cc',
         'common/error_code_utils.cc',
+        'update_engine_client.cc',
      ],
     },
     # server-side code. This is used for delta_generator and unittests but not