docs/devel/tracing: Update trace.h creation rune to include SPDX

docs/devel/tracing: Update trace.h creation rune to include SPDX

checkpatch now checks that new files have an SPDX line. If you use the
shell rune in tracing.rst to create a trace.h wrapper header, this
triggers checkpatch to complain.

Although these files are tiny, it's worth having the SPDX line to
avoid having to add extra exception cases to checkpatch.

Update the rune to include creating an SPDX line.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>