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

tools/testing/selftests/ftrace/test.d/ftrace/func-filter-stacktrace.tc

 #!/bin/sh
-# SPDX-License-Identifier: GPL2.0
+# SPDX-License-Identifier: GPL-2.0
 # description: ftrace - stacktrace filter command
 # flags: instance
 

tools/testing/selftests/ftrace/test.d/ftrace/func_cpumask.tc

 #!/bin/sh
-# SPDX-License-Identifier: GPL2.0
+# SPDX-License-Identifier: GPL-2.0
 # description: ftrace - function trace with cpumask
 
 if ! which nproc ; then

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

 #!/bin/sh
-# SPDX-License-Identifier: GPL2.0
+# SPDX-License-Identifier: GPL-2.0
 # 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.

tools/testing/selftests/ftrace/test.d/tracer/wakeup.tc

 #!/bin/sh
-# SPDX-License-Identifier: GPL2.0
+# SPDX-License-Identifier: GPL-2.0
 # description: Test wakeup tracer
 
 if ! which chrt ; then

tools/testing/selftests/ftrace/test.d/tracer/wakeup_rt.tc

 #!/bin/sh
-# SPDX-License-Identifier: GPL2.0
+# SPDX-License-Identifier: GPL-2.0
 # description: Test wakeup RT tracer
 
 if ! which chrt ; then