media: i2c: max9271: Add header include guards to max9271.h

Protect against multiple inclusions of the max9271.h header with include
guards.

Link: https://lore.kernel.org/linux-media/20220102224803.27463-1-laurent.pinchart+renesas@ideasonboard.com
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>