hw/xen/xen_pt: Add missing license

Commit eaab4d60d3 ("Introduce Xen PCI Passthrough, qdevice")
introduced both xen_pt.[ch], but only added the license to
xen_pt.c. Use the same license for xen_pt.h.

Suggested-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20231114143816.71079-17-philmd@linaro.org>

hw/xen/xen_pt.h

@@ -1,3 +1,13 @@
+/*
+ * Copyright (c) 2007, Neocleus Corporation.
+ * Copyright (c) 2007, Intel Corporation.
+ *
+ * SPDX-License-Identifier: GPL-2.0-only
+ *
+ * Alex Novik <alex@neocleus.com>
+ * Allen Kay <allen.m.kay@intel.com>
+ * Guy Zana <guy@neocleus.com>
+ */
 #ifndef XEN_PT_H
 #define XEN_PT_H