kernel: add another new config symbol for kernel 3.14

It was augmented by CONFIG_USB_GADGET...

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Tim Harvey <tharvey@gateworks.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40986 3c298f89-4303-0410-b956-a3cf2f4a3e73