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/[e-mail]
Signed-off-by: Lance Yang <[e-mail]>
Acked-by: Petr Mladek <[e-mail]>
Cc: "Masami Hiramatsu (Google)" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

MAINTAINERS

···
 M:	Andrew Morton <[e-mail]>
 R:	Lance Yang <[e-mail]>
 R:	Masami Hiramatsu <[e-mail]>
+R:	Petr Mladek <[e-mail]>
 L:	[e-mail]
 S:	Maintained
 F:	include/linux/hung_task.h