[flang][NFC] Update #include and comment.

[kiranchandramohan]

LGTM.

flang/include/flang/Optimizer/Dialect/FIRAttr.h

 //===-- Optimizer/Dialect/FIRAttr.h -- FIR attributes -----------*- 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
 //
 //===----------------------------------------------------------------------===//
+//
+// Coding style: https://mlir.llvm.org/getting_started/DeveloperGuide/
+//
+//===----------------------------------------------------------------------===//
 
 #ifndef OPTIMIZER_DIALECT_FIRATTR_H
 #define OPTIMIZER_DIALECT_FIRATTR_H
 
-#include "mlir/IR/Attributes.h"
+#include "mlir/IR/BuiltinAttributes.h"
 
 namespace mlir {
 class DialectAsmParser;
 class DialectAsmPrinter;
 } // namespace mlir
 
 namespace fir {