plat_linux: fix build

Change-Id: Ie6cae6e313601fee538c3476ba2f22ea9958dab2
diff --git a/firmware/inc/gpio.h b/firmware/inc/gpio.h
index 46ff113..9f4dda8 100644
--- a/firmware/inc/gpio.h
+++ b/firmware/inc/gpio.h
@@ -8,8 +8,6 @@
 #include <stdint.h>
 #include <stdbool.h>
 
-#include <plat/inc/gpio.h>
-
 enum GpioMode
 {
     GPIO_MODE_IN = 0,
@@ -31,6 +29,7 @@
     GPIO_PULL_DOWN,
 };
 
+enum GpioAltFunc;
 typedef uint8_t GpioNum;
 struct Gpio;