PNP: add missing include/linux/pnp.h to MAINTAINERS

include/linux/pnp.h should be covered by the PNP entry in MAINTAINERS,
so add it in there.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>