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 <Jonathan.Cameron@huawei.com>
Acked-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/iio/adc/ad7292.c

@@ -8,6 +8,7 @@
 #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>