MAINTAINERS: Add printk indexing maintainers on mention of printk_index

This will primarily catch new and changed printk_index_subsys_emit
calls, but it's also worth catching changes to other printk indexing
infrastructure outside of kernel/printk/index.c.

This avoids churn due to missing ccs when adding new printk indexes, as
was the case recently for the first round of the XFS printk indexing
patches.

Signed-off-by: Chris Down <chris@chrisdown.name>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/YkRp9IhToTmTnkl7@chrisdown.name