[gardening] add a missing license header

Author: glessard

stdlib/public/core/StringUTF8Validation.swift

@@ -1,3 +1,15 @@
+//===--- StringUTF8Validation.swift ---------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 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
+//
+//===----------------------------------------------------------------------===//
+
 private func _isUTF8MultiByteLeading(_ x: UInt8) -> Bool {
   return (0xC2...0xF4).contains(x)
 }