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>

MAINTAINERS

@@ -548,6 +548,7 @@ Tracing
 M: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
 S: Maintained
 F: trace/
+F: docs/tracing.txt
 T: git://github.com/stefanha/qemu.git tracing
 
 Checkpatch