kernel: add missing symbol for v4.9

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

target/linux/generic/config-4.9

@@ -4374,6 +4374,7 @@ CONFIG_SYSFS=y
 # CONFIG_SYSFS_DEPRECATED is not set
 # CONFIG_SYSFS_DEPRECATED_V2 is not set
 # CONFIG_SYSFS_SYSCALL is not set
+# CONFIG_SYSCON_REBOOT_MODE is not set
 # CONFIG_SYSTEMPORT is not set
 # CONFIG_SYSTEM_DATA_VERIFICATION is not set
 # CONFIG_SYSTEM_TRUSTED_KEYRING is not set