projects:ad4170_iio: Fix build errors on mbed platform #317

Job	Run time
lint	43s
check-versions	2m 6s
build (ad2s1210_iio)	1m 31s
build (ad405x_iio)	1m 20s
build (ad4130_iio)	1m 31s
build (ad559xr_console)	1m 25s
build (ad5754r_iio)	1m 25s
build (ad5770r_console)	1m 23s
build (ad579x_iio)	1m 33s
build (ad590_console)	1m 27s
build (ad5933_console)	1m 31s
build (ad7124_console)	1m 34s
build (ad7124_temperature-measure)	1m 34s
build (ad7134_iio)	1m 20s
build (ad717x_console)	1m 18s
build (ad717x_iio)	1m 24s
build (ad719x_iio)	1m 20s
build (ad7689_iio)	1m 20s
build (ad77681_iio)	1m 31s
build (ad777x_iio)	1m 21s
build (adt7420_console)	1m 20s
build (evb_discovery_firmware)	1m 29s
build (ltc2672_iio)	1m 29s
build (ltc268x_console)	1m 20s
build (nanodac_console)	1m 23s
	35m 38s

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