MAINTAINERS: add Petr as a reviewer of hung task detector

MAINTAINERS: add Petr as a reviewer of hung task detector

Petr has been actively reviewing hung task detector patches lately.  It's
always good to have a fresh pair of eyes, so let's make it official.

I checked with him, and he's happy to be added.

Link: https://lkml.kernel.org/r/20251119110822.46566-1-lance.yang@linux.dev
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Acked-by: Petr Mladek <pmladek@suse.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>