[ARM] S3C: Fix sparse warning from missing 's3c_device_hwmon'

Fix the following sparse warning due to s3c_device_hwmon being
missing from <plat/devs.h>

devs.c:380:24: warning: symbol 's3c_device_hwmon' was not declared. Should it be static?
Signed-off-by: NBen Dooks <ben@simtec.co.uk>
Signed-off-by: NBen Dooks <ben-linux@fluff.org>

arch/arm/plat-s3c/include/plat/devs.h

@@ -34,6 +34,7 @@ extern struct platform_device s3c_device_iis;
 extern struct platform_device s3c_device_rtc;
 extern struct platform_device s3c_device_adc;
 extern struct platform_device s3c_device_sdi;
+extern struct platform_device s3c_device_hwmon;
 extern struct platform_device s3c_device_hsmmc0;
 extern struct platform_device s3c_device_hsmmc1;
 extern struct platform_device s3c_device_hsmmc2;