Use override in all the places where it is possible

This will avoid -Winconsistent-overrides in the future. Done using the
-Wsuggest-override warning of GCC 5.1

BUG=

Change-Id: I707a649dc368f5dd1e139fd144370abcac0b6263
Reviewed-on: https://chromium-review.googlesource.com/294920
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/compiler/preprocessor/DirectiveParser.cpp b/src/compiler/preprocessor/DirectiveParser.cpp
index 7f8f27b..1c53289 100644
--- a/src/compiler/preprocessor/DirectiveParser.cpp
+++ b/src/compiler/preprocessor/DirectiveParser.cpp
@@ -154,7 +154,7 @@
     }
 
   protected:
-    virtual void lex(Token *token)
+    void lex(Token *token) override
     {
         const char kDefined[] = "defined";