[ARM] S3C24XX: Fix sparse error in gpiolib.c

Fix the following sparse error generated by including
<plat/gpio-core.h> instead of <mach/gpio-core.h>

gpiolib.c:78:22: warning: symbol 's3c24xx_gpios' was not declared. Should it be static?

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