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/translator/ValidateLimitations.cpp b/src/compiler/translator/ValidateLimitations.cpp
index f8d2306..ed60c37 100644
--- a/src/compiler/translator/ValidateLimitations.cpp
+++ b/src/compiler/translator/ValidateLimitations.cpp
@@ -35,7 +35,7 @@
     // Returns true if the parsed node represents a constant index expression.
     bool isValid() const { return mValid; }
 
-    virtual void visitSymbol(TIntermSymbol *symbol)
+    void visitSymbol(TIntermSymbol *symbol) override
     {
         // Only constants and loop indices are allowed in a
         // constant index expression.