boards: nrf9160dk_nrf52840: Add missing inclusion

This code uses the `NRF_DT_GPIOS_TO_PSEL` macro, so it should include
`<soc.h>` explicitly and not rely on the header being included by some
other one.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>

boards/arm/nrf9160dk_nrf52840/board.c

@@ -9,6 +9,7 @@
 #include <zephyr/drivers/gpio.h>
 #include <zephyr/devicetree.h>
 #include <zephyr/logging/log.h>
+#include <soc.h>
 #include <hal/nrf_gpio.h>
 
 LOG_MODULE_REGISTER(board_control, CONFIG_BOARD_NRF9160DK_LOG_LEVEL);