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>