ARM: S3C2410: Fix sparse warnings in arch/arm/mach-s3c2410/gpio.c

Fix sparse warning in arch/arm/mach-s3c2410/gpio.c due to missing
include of <mach/gpio-fns.h>. Fixes the following warning:

warning: symbol 's3c2410_gpio_irqfilter' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben-linux@fluff.org>