Add missing OVERRIDE qualifiers in code generators.

Test: mmma art
Change-Id: I91d0a2dc23dc8d63a9bb3607eb1c1517eabaeb1f
diff --git a/compiler/optimizing/code_generator_arm64.h b/compiler/optimizing/code_generator_arm64.h
index f0d7910..f1dc7ee 100644
--- a/compiler/optimizing/code_generator_arm64.h
+++ b/compiler/optimizing/code_generator_arm64.h
@@ -644,10 +644,10 @@
   // artReadBarrierForRootSlow.
   void GenerateReadBarrierForRootSlow(HInstruction* instruction, Location out, Location root);
 
-  void GenerateNop();
+  void GenerateNop() OVERRIDE;
 
-  void GenerateImplicitNullCheck(HNullCheck* instruction);
-  void GenerateExplicitNullCheck(HNullCheck* instruction);
+  void GenerateImplicitNullCheck(HNullCheck* instruction) OVERRIDE;
+  void GenerateExplicitNullCheck(HNullCheck* instruction) OVERRIDE;
 
  private:
   using Uint64ToLiteralMap = ArenaSafeMap<uint64_t, vixl::aarch64::Literal<uint64_t>*>;