selftests/ftrace: Add SPDX License Identifier to template

Add SPDX License Identifier line to template file so
that someone who makes new testcase from the template
does not forgot it.
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/ftrace/test.d/template

 #!/bin/sh
+# SPDX-License-Identifier: GPL2.0
 # description: %HERE DESCRIBE WHAT THIS DOES%
 # you have to add ".tc" extention for your testcase file
 # Note that all tests are run with "errexit" option.