kernel: add missing symbol

Signed-off-by: John Crispin <[email protected]>

target/linux/generic/config-4.14

@@ -2836,6 +2836,7 @@ CONFIG_MTD_SPLIT_SUPPORT=y
 # CONFIG_MTD_TESTS is not set
 # CONFIG_MTD_UBI is not set
 # CONFIG_MTD_UIMAGE_SPLIT is not set
+# CONFIG_MTD_VIRT_CONCAT is not set
 CONFIG_MULTIUSER=y
 # CONFIG_MUTEX_SPIN_ON_OWNER is not set
 # CONFIG_MV643XX_ETH is not set