include/gpio.h: Add 'IWYU pragma: export' comment

This pragma says to IWYU (Include What You Use) that the current file
is supposed to provide commented headers.

Change-Id: Iedd798eebf3376b7631fc9aa1ca0ba92867382bd
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70520
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/gpio.h

@@ -3,7 +3,7 @@
 #ifndef __SRC_INCLUDE_GPIO_H__
 #define __SRC_INCLUDE_GPIO_H__
 
-#include <soc/gpio.h>
+#include <soc/gpio.h> /* IWYU pragma: export */
 #include <types.h>
 
 /* <soc/gpio.h> must typedef a gpio_t that fits in 32 bits. */