Add missing OVERRIDE qualifiers in code generators.

Test: mmma art
Change-Id: I91d0a2dc23dc8d63a9bb3607eb1c1517eabaeb1f
diff --git a/compiler/optimizing/code_generator_x86.h b/compiler/optimizing/code_generator_x86.h
index e225098..04a0a3d 100644
--- a/compiler/optimizing/code_generator_x86.h
+++ b/compiler/optimizing/code_generator_x86.h
@@ -561,9 +561,9 @@
     }
   }
 
-  void GenerateNop();
-  void GenerateImplicitNullCheck(HNullCheck* instruction);
-  void GenerateExplicitNullCheck(HNullCheck* instruction);
+  void GenerateNop() OVERRIDE;
+  void GenerateImplicitNullCheck(HNullCheck* instruction) OVERRIDE;
+  void GenerateExplicitNullCheck(HNullCheck* instruction) OVERRIDE;
 
   // When we don't know the proper offset for the value, we use kDummy32BitOffset.
   // The correct value will be inserted when processing Assembler fixups.