ftrace: Maintain samples/ftrace

There's no entry in MAINTAINERS for samples/ftrace. Add one so that the
FTRACE maintainers are kept in the loop.

Link: https://lkml.kernel.org/r/20230103124912.2948963-2-mark.rutland@arm.com

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>