xen: fix copyright header

Some of the code in xen-locore.S was picked from Cherry G. Mathew
amd64 Xen PV branch, but I've failed to set the proper copyright, so
do it now.

Approved by: gibbs

sys/amd64/amd64/xen-locore.S

@@ -1,8 +1,12 @@
 /*-
  * Copyright (c) 2003 Peter Wemm <[email protected]>
+ * Copyright (c) 2011-2012 Spectra Logic Corporation
  * Copyright (c) 2013 Roger Pau Monne <[email protected]>
  * All rights reserved.
  *
+ * This software was developed by Cherry G. Mathew <[email protected]>
+ * under sponsorship from Spectra Logic Corporation.
+ *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met: