stm class: SPDX-ify the documentation

Add the SPDX header to the STM class documentation.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/trace/stm.rst

+.. SPDX-License-Identifier: GPL-2.0
+
 ===================
 System Trace Module
 ===================