tracing: Add SPDX License format tags to tracing files

Add the SPDX License header to ease license compliance management.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c
index 50f44b7..b0875b3 100644
--- a/kernel/trace/trace_printk.c
+++ b/kernel/trace/trace_printk.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * trace binary printk
  *