selftests/ftrace: Fix invalid SPDX identifiers

While GPL2.0 looks about right, the correct and valid identifiers for GPL v2
only code are 'GPL-2.0' or 'GPL-2.0-only'.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>