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/MacroExpander.cpp b/src/compiler/preprocessor/MacroExpander.cpp
index 69e2f39..9be9bf6 100644
--- a/src/compiler/preprocessor/MacroExpander.cpp
+++ b/src/compiler/preprocessor/MacroExpander.cpp
@@ -26,7 +26,7 @@
         mIter = mTokens.begin();
     }
 
-    virtual void lex(Token *token)
+    void lex(Token *token) override
     {
         if (mIter == mTokens.end())
         {