iio: adc: ad7292: Add explicit include for of.h

With linux/acpi.h no longer implicitly including of.h, add an explicit
include of of.h to fix the following error:

drivers/iio/adc/ad7292.c:307:9: error: implicit declaration of function 'for_each_available_child_of_node'; did you mean 'fwnode_for_each_available_child_node'? [-Werror=implicit-function-declaration]

Acked-by: Jonathan Cameron <[e-mail]>
Acked-by: Marcelo Schmitt <[e-mail]>
Reviewed-by: Jacob Keller <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/iio/adc/ad7292.c

···
 #include <linux/bitfield.h>
 #include <linux/device.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/regulator/consumer.h>
 #include <linux/spi/spi.h>