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/ValidateOutputs.h b/src/compiler/translator/ValidateOutputs.h
index 4ba85d8..06f6399 100644
--- a/src/compiler/translator/ValidateOutputs.h
+++ b/src/compiler/translator/ValidateOutputs.h
@@ -21,7 +21,7 @@
 
     int validateAndCountErrors(TInfoSinkBase &sink) const;
 
-    virtual void visitSymbol(TIntermSymbol*);
+    void visitSymbol(TIntermSymbol *) override;
 
   private:
     int mMaxDrawBuffers;