Turn on build/header_guard in cpplint.

Set the correct root path in CPPLINT.cfg and fix existing violations.

Test: tools/repohooks/tools/cpplint.py --quiet system/update_engine/**/*.h
Change-Id: I37812854a68116c69bcef28f8b15856b89a07c4d
diff --git a/mock_metrics_reporter.h b/mock_metrics_reporter.h
index a0f164b..c35a105 100644
--- a/mock_metrics_reporter.h
+++ b/mock_metrics_reporter.h
@@ -14,8 +14,8 @@
 // limitations under the License.
 //
 
-#ifndef UPDATE_ENGINE_MOCK_METRICS_REPORTER_H
-#define UPDATE_ENGINE_MOCK_METRICS_REPORTER_H
+#ifndef UPDATE_ENGINE_MOCK_METRICS_REPORTER_H_
+#define UPDATE_ENGINE_MOCK_METRICS_REPORTER_H_
 
 #include <gmock/gmock.h>
 
@@ -80,4 +80,4 @@
 
 }  // namespace chromeos_update_engine
 
-#endif  // UPDATE_ENGINE_MOCK_METRICS_REPORTER_H
+#endif  // UPDATE_ENGINE_MOCK_METRICS_REPORTER_H_