Fix text trailing after preprocessor macro warning in util/Event.h

Change-Id: Ie977e6fce0e5e0efeb16f0387bf653bbd2d1cd93
Reviewed-on: https://chromium-review.googlesource.com/269127
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>

util/Event.h

@@ -84,4 +84,4 @@ class Event
     };
 };
 
-#endif SAMPLE_UTIL_EVENT_H
+#endif // SAMPLE_UTIL_EVENT_H