gpio: fix build on CONFIG_GPIO_SYSFS=n

If CONFIG_GENERIC_GPIO=y && CONFIG_GPIO_SYSFS=n, gpio_export() in
asm-generic/gpio.h refers -ENOSYS and causes build error.
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/gpio.h

@@ -2,6 +2,7 @@
 #define _ASM_GENERIC_GPIO_H
 
 #include <linux/types.h>
+#include <linux/errno.h>
 
 #ifdef CONFIG_GPIOLIB