tracing: Add SPDX License format to bpf_trace.c

Add the SPDX License header to ease license compliance management.
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/bpf_trace.c

+// SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2011-2015 PLUMgrid, http://plumgrid.com
  * Copyright (c) 2016 Facebook
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of version 2 of the GNU General Public
- * License as published by the Free Software Foundation.
  */
 #include <linux/kernel.h>
 #include <linux/types.h>