MAINTAINERS: add myself as a tracing reviewer

Since I'm actively involved in a number of arch bits that intersect
ftrace (e.g. the actual arch implementation on arm64, stacktracing,
entry management, and general instrumentation safety), add myself as a
reviewer of the core ftrace code so that I have the change to catch any
potential problems early.

I spoke with Steven about this at LPC, and it seemed to make sense to
add me as a reviewer.

Link: https://lkml.kernel.org/r/20220928114621.248038-1-mark.rutland@arm.com

Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>