staging: iio: resolver: ad2s1210: Add explicit include for of.h

staging: iio: resolver: ad2s1210: 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/staging/iio/resolver/ad2s1210.c:706:21: error: implicit declaration of function 'of_match_ptr' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.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>