kernel: add missing config symbol (#19870)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r45992

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@45993 3c298f89-4303-0410-b956-a3cf2f4a3e73