ftrace: Create separate entry in MAINTAINERS for function hooks

The function hooks (ftrace) is a completely different subsystem from the
general tracing. It manages how to attach callbacks to most functions in
the kernel. It is also used by live kernel patching. It really is not part
of tracing, although tracing uses it.

Create a separate entry for FUNCTION HOOKS (FTRACE) to be separate from
tracing itself in the MAINTAINERS file.

Perhaps it should be moved out of the kernel/trace directory, but that's
for another time.

Link: https://lkml.kernel.org/r/20221006144439.459272364@goodmis.org

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