kernel/3.10: add missing config symbol

Based on 'kernel: update x86 generic to 3.10.13' patch
from http://patchwork.openwrt.org/patch/4129/



Signed-off-by: Ulrich Weber <uw@xyne.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 38498