power/supply: fix sc27xx_fuel_gauge build errors

Fix build errors when FUEL_GUAGE_SC27XX is enabled but IIO is either
not enabled or IIO=m and FUEL_GUAGE_SC27XX=y.

ld: drivers/power/supply/sc27xx_fuel_gauge.o: in function `sc27xx_fgu_get_temp':
sc27xx_fuel_gauge.c:(.text+0x1d2): undefined reference to `iio_read_channel_processed'
ld: drivers/power/supply/sc27xx_fuel_gauge.o: in function `sc27xx_fgu_probe':
sc27xx_fuel_gauge.c:(.text+0x97f): undefined reference to `devm_iio_channel_get'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-pm@vger.kernel.org
Acked-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>