maintainers: Add docs/tracing.txt to Tracing The topic of whether and by whom docs/tracing.txt is maintained was brought up. It currently does not have an official maintainer. Add it to the tracing section so that Stefan gets cc'ed on patches. Signed-off-by: Andreas Färber <afaerber@suse.de> Acked-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Cc: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>