iio: proximity: cros_ec_mkbp_proximity: Switch to including mod_devicetable.h for struct of_device_id definition

of.h was only included for this definition, so include the correct header
instead.
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://patch.msgid.link/20240823220718.743596-1-andy.shevchenko@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>