[NFC] add missing liscence to file

Author: Finn Plummer

clang/lib/Lex/LexHLSLRootSignature.cpp

@@ -1,3 +1,11 @@
+//=== LexHLSLRootSignature.cpp - Lex Root Signature -----------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
 #include "clang/Lex/LexHLSLRootSignature.h"
 
 namespace clang {