[flang][NFC] Update #include.

flang/include/flang/Optimizer/CodeGen/CodeGen.h

 //===-- Optimizer/CodeGen/CodeGen.h -- code generation ----------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef OPTIMIZER_CODEGEN_CODEGEN_H
 #define OPTIMIZER_CODEGEN_CODEGEN_H
 
+#include "mlir/IR/BuiltinOps.h"
 #include "mlir/Pass/Pass.h"
 #include "mlir/Pass/PassRegistry.h"
 #include <memory>
 
 namespace fir {
 
 struct NameUniquer;