Use the new licence header in ExtraTests's cpp files

tests/ExtraTests/X90-WindowsHeaderInclusion.cpp

@@ -1,3 +1,11 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE_1_0.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
 // X90-WindowsHeaderInclusion.cpp
 // Test that the Catch2 header compiles even after including windows.h
 // without defining NOMINMAX first. As an FYI, if you do that, you are