boot: zephyr: select GPIO when MCUBOOT_SERIAL is enabled

Select GPIO when MCUBOOT_SERIAL is enabled.

Signed-off-by: Emanuele Di Santo <emdi@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index 2ce1b2d..34cb541 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -143,8 +143,9 @@
 	bool "MCUboot serial recovery"
 	default n
 	select REBOOT
-	select UART_INTERRUPT_DRIVEN
+	select GPIO
 	select SERIAL
+	select UART_INTERRUPT_DRIVEN
 	select BASE64
 	select TINYCBOR
 	help