Update build.ps1

Add license header

Author: compnerd

utils/build.ps1

@@ -1,3 +1,15 @@
+#===--- build.ps1 - Windows Build Script -----------------*- PowerShell -*-===//
+#
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See https://swift.org/LICENSE.txt for license information
+# See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+#
+#===----------------------------------------------------------------------===//
+
 # Copyright 2020 Saleem Abdulrasool <[email protected]>
 # Copyright 2023 Tristan Labelle <[email protected]>