um: Add SPDX headers for files in arch/um/drivers

Convert files to use SPDX header. All files are licensed under the GPLv2.

Signed-off-by: Alex Dewar <alex.dewar@gmx.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/slirp_user.c

@@ -1,6 +1,6 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (C) 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
- * Licensed under the GPL.
  */
 
 #include <unistd.h>