[SYCL][NFC] Add licence header to iostream_proxy (#10399)

Author: bader

sycl/include/sycl/detail/iostream_proxy.hpp

@@ -1,4 +1,13 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
 #pragma once
+
 #include <istream>
 #include <ostream>